Submitted by taracus t3_ygfptx in askscience

Since dark matter (seemingly) only interact through gravity, is there any reason it's angular momentum would line up with the rest of matter?

I'm under the impression that the reason all planets spin the same way around the sun and all the stars spin the same way around the galaxy center is because of collisions with has "evened out" the angular momentum to some average?



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Aseyhe t1_iu8lk8c wrote

Since they are both subject to the same gravitational forces, dark and ordinary matter at the same location orbit at similar average speeds. However, dark matter has a much broader distribution of velocities, both in magnitude and direction.

Basically, ordinary matter is able to cool via inelastic collisions, causing it to lose energy but not angular momentum. Thus it tends to settle into configurations that reduce the ratio of energy to angular momentum, like disks. Note that collisions alone don't suffice for this; energy loss is needed. Within a disk, particles have largely coherent velocities with only a small spread. For example, material within our section of the disk orbits at roughly 220 km/s, but its velocity dispersion is only in the tens of km/s. (The velocity dispersion is the root-mean-squared deviation from the average velocity.)

In contrast, dark matter has no coherent motion, instead moving in random directions with a wide spread of speeds. The local dark matter velocity dispersion is something like 270 km/s.

(It should be noted, though, that many galaxies don't have disks. Only gas cools; stars are essentially collisionless, just like dark matter. So for example, if a galaxy's mass gets significantly redistributed, perhaps due to a merger, after it has converted its gas into stars, then the stars will not reform a disk.)


elite4koga t1_iu8saa5 wrote

This answer is making some unstated assumptions about theoretical dark matter. I think for clarity it would be helpful to write out those assumptions here. Particle nature, certain mass range etc.

Dark matter has never been directly observed, to my knowledge there isn't even strong evidence of where its located. So the certainty of statements like "dark matter has no coherent motion" I think needs some caveats.


Aseyhe t1_iu8tdpt wrote

Sure, it holds for almost every dark matter model that we consider, including

  • cold dark matter
  • warm dark matter
  • fuzzy dark matter (which is so light that quantum effects are important)
  • primordial black holes or other massive objects
  • self-interacting dark matter (with elastic collisions)

There are, however, "dissipative" dark matter models that are capable of losing energy through inelastic collisions. In those models, there would be a tendency to form disks over very long time scales. Since we know that dark matter halos are much more broadly extended than galaxies, the time scale for dark disk formation must be a lot longer than the time scale to form galactic disks, though.

Also, a dark matter particle that has a significant nongravitational interaction with ordinary matter could be dragged along with the disk. It's hard to reconcile this possibility with (1) absence of dark matter direct detection and (2) absence of dark matter creation in colliders, though.

> to my knowledge there isn't even strong evidence of where its located.

We have a fairly precise picture, e.g.


elite4koga t1_iu94rbt wrote

This is great info, exactly what I was looking for. Thanks for the follow up!


TastiSqueeze t1_iu9ia3a wrote

In some cases dark matter has been separated from the galaxy it was associated with and in a few cases is believed to rotate in the opposite direction as the host galaxy.


Goodpie2 t1_iua7ga5 wrote

How do we even know that?


ViscountTinew t1_iuagdu0 wrote

Gravitational lensing is one way - even if we can't see the dark matter, we can notice the lensing of light passing around it if there's enough of the stuff.

The obvious example is the Bullet Cluster. It's two galaxies that collided with and passed through each other. It's been a while since the collision so there's a fair amount of space in between the two galaxies.

What's interesting about the bullet cluster is that the lensing effects appear to be focused around a point in between the two galaxies, in what appears to be empty space, rather than the galaxies themselves, implying a large invisible mass of dark matter that has been left behind since the collision.


PM_Me_Frosted_Tits t1_iuar47i wrote

How do we know that the mass is, in fact, dark matter?

What methods do they have at their disposal ruled out, say... A large field of interstellar medium that has coalesced since the event and wasn't dispersed by it for some reason unknown to the current model of galactic collisions?

From my understanding we haven't technically proven dark matter's existence yet. We see the effects of something and use the term "dark matter" to explain it, but do we know for sure that it isn't a secondary effect of something we can currently detect?


ViscountTinew t1_iub3gg1 wrote

Because whatever the mass is, it isn't emitting any light or blocking any light. Everything has a temperature and normal matter should be detectable in some wavelength or another, especially in the amount needed to create the lensing effects. A cloud of dust and gas massive enough to cause the lensing would be easily visible in the radio/far-infrared spectrums.


PM_Me_Frosted_Tits t1_iubbkah wrote

I figured there was some reaction I just wasn't aware of that they've used, I didn't even think of things like friction between molecules as being enough to detect against the background of space at that distance.

I appreciate it, this is what I was hoping someone would be able to explain.


Natural_Pressure6329 t1_iubfq4w wrote

This brings a question to my mind. There is a “neutral” point between two masses, it’s the point at which the two gravitational forces sort of level out (I forgot the proper name), could this be the cause of the lensing being centered there?


ryry1237 t1_iubg437 wrote

Presumably any earlier calculations would already be done with this in mind and the math doesn't add up with just ordinary matter.


Putnam3145 t1_iuayg6d wrote

Interstellar medium undergoes more friction than our observations would allow.


PM_Me_Frosted_Tits t1_iub1rf9 wrote

Meaning we'd see something in the Infrared range because of heat produced during friction?


forte2718 t1_iu93rqv wrote

>Dark matter has never been directly observed, ...

I'd like to point out here that the known neutrinos qualify as (hot) dark matter, and other than their very light mass / relativistic speeds, they have essentially identical properties to the cold dark matter that we haven't yet directly observed. So from a definitions standpoint, we have in fact directly observed a form of dark matter (and we regularly produce it in labs for the purpose of studying it). The reason I mention this fact is because people seem to have a common misconception that dark matter is some kind of farfetched, esoteric hypothesis that makes it different from everything that we actually do know, when the reality is that it's incredibly similar to some of the things that we do already definitely know — so similar in fact that one of the leading hypotheses for a cold dark matter particle candidate is in fact a heavy neutrino.

>... to my knowledge there isn't even strong evidence of where its located. So the certainty of statements like "dark matter has no coherent motion" I think needs some caveats.

I'd now also like to point out just how overwhelming the evidence for dark matter is. Contrary to what you've said, we do in fact have multiple ways to measure the bulk properties and behavior of dark matter with enough sensitivity to narrow in on its locations, densities, velocity distribution, speed distribution, etc. This sort of evidence comes from about a dozen completely independent metrics:

  • Galaxy rotation curves
  • Velocity dispersions of stars in galaxies
  • Gravitational lensing and microlensing surveys
  • X-ray spectra from gas-heavy galaxy clusters
  • The angular power spectrum of the CMB
  • The matter power spectrum and rates of structure formation in the early universe
  • Observations of more than a dozen colliding galaxy clusters, including the Bullet Cluster, Abell 520, NGC 1052-DF2, and many others
  • Reconciling type Ia supernova distance measurements with the observed flatness of the universe's shape
  • Galaxy redshift surveys
  • Spectral measurements of the Lyman-Alpha forest for distant galaxies

... and there are even more things not mentioned in this list as well. What really drives the nail home is that all of these completely independent metrics are all in close agreement as to exactly how much dark matter there is, how it is distributed, and what most all of its bulk properties and behavior are. When you have such a wealth of evidence where all of it is unequivocably pointing toward the same explanation ... there's just no reasonable excuse to deny it.

It may be true that we haven't directly detected individual cold dark matter particles, but that doesn't mean we haven't detected the direct consequences of the existence of cold dark matter in bulk. The various observational signals supporting the existence of cold dark matter are clear, robust, and all consistent with each other. Among honest researchers as well as honest laypeople who have an interest in science, there really can be no doubt that dark matter exists and has all of the properties that we've undisputably discovered it to have.

Edit: think about it like this. Many hundreds of years ago, people didn't know what the mechanism underlying fire was. Some people postulated the existence of a light, flammable fluid inside most forms of matter called phlogiston; others sought out different alchemic explanations. At the time, nobody knew what fire actually was at a microscopic level; they didn't know about oxygen, combusion, or chemistry in general. But do you think anybody working on the problem at the time would have denied the existence of fire? Of course not — all you had to do was rub some sticks together to prove that fire clearly existed! You don't need to have a microscopic understanding of fire in order to know with certainty that fire exists, because you can clearly see the bulk behavior of fire with your own eyes. The situation is similar with dark matter, today: we may not know what its microscopic description is yet, but we can see the macroscopic evidence very clearly with our various telescopes and instruments that are designed for making precision measurements of the cosmos. The evidence we see for dark matter's existence is quite clear and undeniable.


menntu t1_iu99t8u wrote

Krikey, mate - you are a refreshing trove of superb information. Truly appreciate the explanations.


forte2718 t1_iu9bgab wrote

Thanks, I'm glad you found it helpful. :) Cheers!


elite4koga t1_iu973f3 wrote

I wasn't attempting to refute dark matter as a particle or come off as a MOND supporter, I just wanted the poster to provide more info about the knowns vs unknowns when responding to the question.


forte2718 t1_iu995o2 wrote

That's fine. I was just responding to try and correct some of the misconceptions in what you said — about dark matter never being directly observed (it has), about us not having strong evidence about its location (which we do), about the certainty of statements about dark matter's other properties, etc.

A lot of this info is outlined in encyclopedia articles like Wikipedia's, so it's pretty accessible. We get a lot of boneheads who won't even do that tiny amount of research before insisting that dark matter is some kind of hoax or fudge factor, so apologies if I stomped on your foot there, that wasn't my intention. I was just trying to respond to the things you said and point out that they aren't correct, lest those misconceptions propagate and get even further out of control. :(


Throwaway_97534 t1_iu9gtyo wrote

If we were in space at relative rest and there was an earth-mass chunk of dark matter that wandered into our path, does that mean we would suddenly find ourselves falling toward it for seemingly no reason?

I'm assuming we haven't observed dark matter at smaller than galactic scales, but I'm wondering if the current theories and observations allow for smaller amounts as well.

Can we run into planet-sized bits of dark matter just like we can run into planet-sized primordial black holes? One of the theories is that we haven't observed a 9th planet in the solar system that's shepherding trans-neptunian objects because it may actually be a primordial black hole... Could it also be a small bit of dark matter?


forte2718 t1_iu9iwcn wrote

>If we were in space at relative rest ...

At relative rest with respect to what? Remember ... all speeds are relative to other things! :)

>... and there was an earth-mass chunk of dark matter that wandered into our path, ...

Dark matter doesn't really clump up the way ordinary matter does. The only way you would get this is if dark matter happens to be in the form of "MACHOs" (massive compact halo objects) such as black holes, but that hypothesis has almost been completely ruled out except for a range of masses that excludes things anywhere near the Earth's mass. So, this just isn't an actual possibility, I'm afraid!

>... does that mean we would suddenly find ourselves falling toward it for seemingly no reason?

We would feel its gravitational effects, yes. If you had something like an Earth-mass MACHO, it could do things like disrupt orbits, to the extent that something of Earth mass can do so (naturally something like the Sun or Jupiter wouldn't be significantly affected, but smaller planets would).

>I'm assuming we haven't observed dark matter at smaller than galactic scales, but I'm wondering if the current theories and observations allow for smaller amounts as well.

Unfortunately, not really ... at least, not also in grouped clumps that are within a few orders of magnitude of the Earth's mass. You can have smaller amounts if it is very diffuse (like axions or neutrinos or some other particulate form that doesn't interact electromagnetically) but not if it clumps up due to an interaction of some kind.

>Can we run into planet-sized bits of dark matter just like we can run into planet-sized primordial black holes?

That's a hard no, unfortunately. The observational evidence (at least that which I am aware of) excludes as a form of dark matter MACHOs including primordial black holes in the mass range of 10^(-8) Earth masses or higher. [1]

>One of the theories is that we haven't observed a 9th planet in the solar system that's shepherding things because it may actually be a primordial black hole... Could it also be a small bit of dark matter?

It and things like it couldn't be a significant form of dark matter, no. I don't see why it couldn't be a black hole though, whether primordial or otherwise.

Hope that helps,


Victra_au_Julii t1_iuaj791 wrote

>about dark matter never being directly observed (it has),

Do you mean a type of dark matter has been observed? That doesn't mean this particular observation is the dark matter that is responsible for the differences in our calculations - measurements for galaxies.


forte2718 t1_iuao3ig wrote

>Do you mean a type of dark matter has been observed?

Yes as I explained two posts prior, we have observed dark matter directly — three types of dark matter, in fact: the electron neutrino, muon neutrino, and tau neutrino.

>That doesn't mean this particular observation is the dark matter that is responsible for the differences in our calculations - measurements for galaxies.

Yes, I explained that too, also in the second to previous post. Perhaps you should go back and read it!


Victra_au_Julii t1_iubkfor wrote

I was being tongue in cheek. No we have not observed dark matter, as in, the dark matter that makes the massive difference in mass observed in the universe. Those things you mentioned are not what makes up most Dark Matter.


scientist4321 t1_iua04as wrote

Part of the hesitation is that we’ve been down this road before with “ether”, in pre-relativistic times. It was nearly consensus that ether existed and was necessary to explain light and radio, but unfortunately nobody was able to find it. And then those pesky experiments about the speed of light, and the kid from the patent office.

I’m far from an expert, and dark matter seems from what you write proven by many independent sources. But, I can guess, for many, it smells like an esoteric substance stubbornly refusing to be directly identified.


forte2718 t1_iua84oa wrote

The difference between aether and dark matter is that with aether theories, experiment after experiment showed repeatedly that it didn't exist, or at least didn't work the way it was supposed to. There was Fizeau's experiment in 1851 which largely ruled out any aether drag effects, the Michelson—Morley experiment in 1887 which established the constancy of the speed of light and ruled out the aether wind hypothesis, the Trouton–Noble experiment in 1903, the Rayleigh and Brace experiments between 1902 and 1904, ... there was a wealth of experimental evidence available showing the problems with aether models. Also it wasn't "nearly consensus" that aether existed, there was quite a lot of debate all throughout that period as to whether light was a wave (one that travelled in a presumed medium — an aether of some sort) or whether light was a particle or "corpuscule" (and hence a form of matter that did not require a medium within which to propagate). Various people proposed either/both wave- or particle-based theories of light and experiments continued to both confirm and refute aspects of each proposed model, all the way from their inception into the early 20th century. The problems with both kinds of models only really began to start being resolved with the advent of early quantum mechanics and the emergence of wave-particle duality as a feature in physics.

With the dark matter situation however, you have dozens of kinds of observations that are all in general agreement about dark matter, while those same observations are largely contradicted by the predictions of modified-gravity models. Unlike with the aether, this isn't a situation where nobody's model fits all the data; with dark matter there is a clear matter-based model that does fit all of the data, and then a bunch of alternative models that don't. That's a pretty huge difference.

>I’m far from an expert, and dark matter seems from what you write proven by many independent sources. But, I can guess, for many, it smells like an esoteric substance stubbornly refusing to be directly identified.

And this is the crux of the public relations issue on this topic: really only the people who are "far from experts" who aren't actually familiar with all of the evidence feel that way. Among actual experts, the consensus is that dark matter exists and that modified gravity models don't actually work. The only people who really have a problem with dark matter anymore are the ones who are uninformed about it. :(


throw_every_away t1_iuau5oh wrote

I always thought that we only knew dark matter existed because of the space where we knew it “should” be. I didn’t know we had models of its actual makeup. That’s pretty dang cool, when did this happen?


forte2718 t1_iub3tz8 wrote

Uhhh ... it's been happening for basically as long as we've known dark matter existed? The first real evidence for dark matter started coming in almost 100 years ago, in the 1930s and 1940s. The earliest dark matter models were overly simple — just treating galaxies as if they had extra mass — and they had various problems. Throughout the following decades, models of the cosmos became increasingly sophisticated ... and increasingly conflicted, as every model had some seemingly irreconcilable problems (regardless of dark matter) so it wasn't clear which model was the correct, or even the closest to correct. As I understand it, during the early 1980s the first modern models of dark matter were proposed, with distributions not matching those of baryonic matter. And with the discovery of the accelerating expansion of the universe and the need for a cosmological constant or some other form of dark energy, in the late 1980s / early 1990s, enough evidence had come in for cosmologists to hone in on a model that resolved all the issues with the previous competing models — this model is known the Lambda-CDM model (lambda is the symbol for the cosmological constant term in the Einstein field equations, and CDM stands for "cold dark matter"); it is also known as the "concordance model" because it gracefully resolved all the outstanding problems with the previous models of cosmology, and fit the data much better than any of those other models. Since that time, this model has become known as the "standard model of cosmology" and has stood out as effectively the only model that actually works and fits all of the data. Dark matter has been a part of that model since its inception in the early 90s, and in the three decades since all sorts of tweaks, additions, and parameterizations of that model (including its dark matter aspect) have been explored ... as well as many alternative models, none of which have panned out and found success at fitting all of the data well.

So dark matter has been an accepted part of the modern model of cosmology for at least 3 decades and we've had all manners of more complicated models (and attempts at alternative models) developed during that time. All sorts of supercomputing simulations of structure formation in the cosmos have been run and their statistics compared to observational datasets, with gradual refinement of the narrower strokes as new data has come in from missions like WMAP and the Planck spacecraft.

In summary, it's been happening this entire time because that's what cosmologists do — that's their job. It's not like they've been slacking off for half a century; there are tens of thousands of cosmologists and astrophysicists worldwide who have been working on it doing formal research and experimentation/observation for their entire professional careers. :)


Tupcek t1_iucdh83 wrote

may I ask, is there a lot of dark matter in our solar system, or anywhere near, so we can study it up close in the coming decades or centuries?


forte2718 t1_iud50dw wrote

There isn't a lot, no. Dark matter is very diffuse; in all likelihood there is some streaming through the planet every second of every day, but since it doesn't really interact with baryonic matter much if at all (similar to how neutrinos don't), studying it would be exceedingly difficult. There have been many experiments similar to current neutrino detectors looking for dark matter interactions but none have been found to date.


bmrheijligers t1_iu9ji01 wrote

I love the thoroughness of your deconstruction. The point i believe that was trying to be made is that the name dark matter implies the assumption that it must be some kind of matter. An understandable one, as so far we know only of gravity associated with matter.

An assumption non the less.


forte2718 t1_iu9kmzi wrote

Well, if it makes you feel any better, we've also explored every other type of possibility that is allowed:

  • scalar field models, like the Higgs field
  • vector field models, like electromagnetism and the strong and weak interactions
  • tensor field models, like general relativity and modifications of it
  • supersymmetric matter models

These, together with ordinary matter models, are exhaustive of what the allowed possibilities are, at least for particle forms of dark matter (keeping in mind that aggregate forms, such as black holes and other MACHOs, have also been largely ruled out except for within a range of very small masses). There are also combinations of these, such as TeVeS (which gets its name from combining tensor, vector, and scalar fields).

To date, the only types of models that have succeeded at explaining all of the observational evidence are matter-based models (and possibly some of the more contrived supersymmetric matter models, I am not super familiar with those but I assume they operate similarly to matter models — what I do know is that the most minimal supersymmetric models have been observationally ruled out and to date there remains zero evidence for supersymmetry in nature ... though not for lack of trying; discovering supersymmetric particles was one of the goals of the LHC, as well as other experiments).

So, it's not so much an assumption, so much as "we've explored all of the other possibilities and this is the only possibility that actually works."

XKCD wrote a comic about this. There seems to be this popular misconception that we just assumed it was matter because that's simple. No, we "assume" it is matter because virtually every other possibility has been heavily explored and every single one of them has failed to fit all the data even in the most contrived circumstances. Dark matter is really the only game in town, it is the only model of all the thousands that have been explored which fits all of the data.


Great-Profession689 t1_iu9z3lu wrote

>It may be true that we haven't directly detected individual cold dark matter particles, but that doesn't mean we haven't detected the direct consequences of the existence of cold dark matter in bulk.

It may be true what we haven't yet explained the bullet cluster, but that doesn't mean we haven't detected direct consequences of the existence of modified gravity :)


forte2718 t1_iua9lx1 wrote

The difference is that the Bullet cluster is an observation that needs to be explained for you to have a viable model of the cosmos. Any model which cannot explain it is not viable.

Dark matter models can explain it ... and also can explain all of the other observational data. Dark matter models are viable.

There is no known model of modified gravity that is viable. Every one that has been proposed to date is either too vague to make novel testable predictions (in which case you can't really call it a model to begin with), or it has made novel testable predictions that were found to be in conflict with observations, leading to the proposed model being falsified.


Astrokiwi t1_iu917n4 wrote

It's kind of the other way around - these are the observational constraints on what dark matter has to be, assuming they are governed by known physics.


HunkyMump t1_iu9894h wrote

A side question: because dark matter interact with gravity, can we then infer that dark matter particles that do interact with gravity, are limited to the speed of light in the same way regular matter does?


Fewluvatuk t1_iu9dxfd wrote

We don't have to assume. The speed of causality applies to everything in the universe regardless of its properties.


MrMojorisin521 t1_iu9jtmb wrote

>In contrast, dark matter has no coherent motion, instead moving in random directions with a wide spread of speeds.

You have a source for this? I didn’t know that we had this much knowledge of dark matter. I’m fascinated.


Aseyhe t1_iua2p69 wrote

To be clear, we don't directly measure dark matter velocities; there are several theoretical steps along the chain. See this reply. I'm not sure of a good pedagogical source right now, but here's an academic one (see e.g. section 4).


Belzebutt t1_iu9vvab wrote

You talk about the velocities of dark matter. How DO we measure the velocity of dark matter?


Aseyhe t1_iua0nsa wrote

Good question! It's indirect. For example, we can measure the circular orbit velocities of stars. Dark matter must have the same circular orbit velocity (i.e. velocity of a dark matter particle that is on a circular orbit), because it's subject to the same gravity. That's not yet the velocity dispersion, but it's close.

Next, we have observational evidence (e.g. rotation curves, lensing) that dark matter halos extend much farther than galaxies. This suggests that unlike the ordinary matter, the dark matter cannot efficiently cool -- otherwise it would condense into the galaxies as well. In fact the dark matter halos around large galaxies are consistent with what we find in simulations of dark matter that only interacts gravitationally. This suggests that dark matter is effectively collisionless in this context.

Since dark matter halos form by nearly isotropic collapse and accretion -- dark matter comes in from all directions -- their net angular momentum is small. Thus they should have very little net rotational motion and almost all random motion. This is also what the same simulations tell us.

The specific number "270 km/s" was a quick estimate I made by taking the isothermal sphere model, which is a good approximation for galactic halos over a pretty wide range of radii, and noting that its velocity dispersion is sqrt(3/2) its circular velocity. The local circular velocity is known to be 220 km/s, so that yields 270 km/s.


littleboymark t1_iuae5b9 wrote

So dark matter doesn't collide with itself?


Aseyhe t1_iuafr56 wrote

It could -- that's the self-interacting dark matter idea -- but not at a level that is important in large galactic halos. In fact, I've seen recent suggestions that in the context of self-interacting dark matter, observations favor a velocity-dependent interaction strength that scales as v^(-4), i.e. decreases very strongly with velocity. This would make interactions irrelevant in clusters and large galaxies (which have very high velocity dispersions) and most relevant toward the centers of the smallest galaxies.


nooki1 t1_iu8rgu2 wrote

Maybe I misunderstood you but should regular mass drag any dark energy with it(or other way around)? Loosing or adding momentum due to gravity.


Aseyhe t1_iu8tn7l wrote

Ordinary matter and dark matter both exert gravitational forces on each other, so they influence each other collectively. However there is essentially no momentum exchange between individual particles. Such gravitational collisions are possible (e.g. the slingshot effect), but their importance scales inversely with the number of particles in the system. In the context of galaxies, the individual dark matter particles and ordinary matter particles (whether they be stars or atoms) are far too numerous for gravitational collisions to be important.

(Incidentally, you said dark energy, not dark matter. In case that was intentional, I'll note that we currently have no evidence that dark energy is capable of clustering, but that is an active research topic.)


gnorty t1_iu96i41 wrote

>ordinary matter is able to cool via inelastic collisions, causing it to lose energy but not angular momentum

This seems strange. Where does the energy go? My instinct suggests it is converted to heat but you seem to suggest the opposite.


Aseyhe t1_iu98er6 wrote

It can be radiated away as light.

Collisions involving molecules can excite vibrational or rotational modes, which is probably what you are getting at. Such modes gradually decay by emitting light. Collisions could also excite electronic states, which also later decay by emitting light.


MrMojorisin521 t1_iu9jet2 wrote

In breaking up upon collision probably. Like two cars smashing into each other and deforming.


Insane212 t1_iu9965k wrote

Follow up question: if dark matter is everywhere, is it on planet earth? If yes why have we not been able to study it


Aseyhe t1_iu9ak8w wrote

If dark matter is a particle, then it's constantly passing through the earth, very similarly to neutrinos. Without nongravitational interactions, it can't really get trapped in (or on) the earth. But we are conducting an array of searches for this dark matter. We haven't found anything, which suggests that dark matter must interact very weakly with ordinary matter.

(If dark matter is massive, e.g. primordial black holes, then it's sparse enough that collisions with the earth are extremely rare. Then we can say that there is no dark matter in/on the earth.)


banuk_sickness_eater t1_iu9d2wn wrote

Can you expound on the primordial black holes point?


Aseyhe t1_iu9g5yj wrote

Dark matter's local density is about 0.4 GeV/cm^(3), which is about 10^-25 times the average density of the earth. So for example, if the dark matter were earth-mass black holes, they would reside inside the earth only 1/10^25 of the time, on average. Even asteroid-mass black holes (~10^20 grams = 10^-8 earth masses) would reside inside the earth only 1/10^17 of the time.

At typical velocities (200-300 km/s), a black hole would pass through the earth in ~30 seconds. If the dark matter were black holes of mass 10^20 grams, they would thus encounter the earth roughly every 10^17 * 30 seconds = 100 billion years, which is longer than the age of the universe.


TurtleStudios t1_iu9h9h0 wrote

Haven't microlensing studies ruled out the idea of a halo of primordial black holes around the galaxy?


Aseyhe t1_iu9k5lj wrote

Yes for earth-mass black holes, but no for the asteroid-mass range. Also, microlensing constraints are sensitive to the degree to which the black holes are clustered, which is a topic of ongoing study.


enmacdee t1_iucmm9v wrote

Unrelated question. But how can you have a black hole of mass earth or asteroid. Isn’t the whole idea of a black hole that the gravity is so strong it bends light. If something only has the same mass as the earth how is it able to bend light? Thanks!


Aseyhe t1_iucns8k wrote

You can make almost anything a black hole if you compress it small enough. If you compressed the earth down to about a centimeter, it would become a black hole. For a 10^20 gram asteroid, the relevant size is under a nanometer.


Ripcord t1_iuakv3j wrote

Why couldn't gravitational interactions trap it?


Aumuss t1_iuamhj6 wrote

We think of the earth as gravitationally bound together, and it is, but, what gravity does is "pull". That's it, it "pulls you in a direction".

What stops you, are the other forces.

Normal matter just stops when it encounters other matter.

It can't pass through.

So, gravity pulls everything together, but the fact the stuff interacts means it clumps. It forms a big ball.

But dark matter doesn't stop when it hits the surface of the earth. Or even when it hits the middle, it just keeps going.

In fact, it just goes straight through the earth, as if the earth isn't even there.

The force of gravity isn't strong enough to hold fast moving particles that don't bang into things.

They just float away.


Aseyhe t1_iuam3ie wrote

The first problem is that typical dark matter particles are moving at ~300 km/s with respect to the earth. But even if one particle was very fortunate and fell toward the earth from essentially zero relative velocity, the problem is conservation of energy. The particle would gain speed as it fell, pass through the earth, and then lose the same amount of speed on the way out of the system, escaping earth's influence again.

In principle a particle could be temporarily trapped in the earth's influence via an interaction with the moon, so that it would transfer its energy to the moon. However this still leaves it on an orbit that takes it at least as far as the moon, and it would eventually be ejected by another interaction with the moon. (This sometimes happens with solar system objects.)


YawnTractor_1756 t1_iubqn2v wrote

We haven't found anything, which suggests that dark matter must interact very weakly with ordinary matter might not even be a real thing.


ensalys t1_iu9cv7r wrote

Depends on what you mean with "on Earth". We don't have an invisible mountain of dark matter somewhere on the planet. Considering it doesn't act on electromagnetism, it doesn't really collide, for the most part it would just go through Earth. But yes, dark matter is expected to be present in the solar system, though in tiny amounts. So occasionally some dark matter will go through Earth. How much is unknown, we'll need to a good understanding of what particles make up dark matter.

> If yes why have we not been able to study it

We try, but it's rather difficult due to it no interacting (often) with detectors.


Kered13 t1_iuaw5ft wrote

> So occasionally some dark matter will go through Earth. How much is unknown

It's actually pretty easy to estimate. Since we know the average density of dark matter in the galaxy, and we know that it's essentially uniformly distributed (it doesn't clump up like regular matter), we know that the density is the same around Earth. It's negligible compared to the mass of the earth, but enough that you can assume there is constantly dark matter passing through your body.


ensalys t1_iucexdk wrote

Sure, you can do that. However, to really estimate how often that happens, we need to know the mass of individual bits/particles of dark matter.


andreasbeer1981 t1_iu9eeld wrote

"dark" means, there is no way to observe it, except for its gravitational effects.


exscape t1_iu9n8nz wrote

Not true; if it were, why would so many different teams of scientists spend a ton of money and effort on building dark matter detectors (that aren't based on gravitational detection)?

Dark means it rarely or weakly (not necessarily never, how could we prove such a negative definitely?) interacts electromagnetically.


Nieshtze t1_iua3g98 wrote

And they haven't found a single piece of direct evidence for a massive particle yet, only neutrinos. Are there good reasons to believe that they have a non-zero interaction with electromagnetic field?


andreasbeer1981 t1_iu9upff wrote

"Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not absorb, reflect, or emit electromagnetic radiation and is, therefore, difficult to detect."


exscape t1_iu9vyka wrote

> does not appear

That matches what I said.

And regardless, EM and gravity aren't the only forces in existence. There's no proof it ONLY interacts gravitationally as you said.



andreasbeer1981 t1_iuadk1d wrote

well, there is no proof that dark matter even exists. this is why it makes sense to make all kinds of experiments to find out more.


CrashCalamity t1_iuc7p4y wrote

Consider: Dark matter is something of a "placeholder name" as it pertains to a theory to explain certain anomalous (yet repeatably measureable) results in astrophysics. Something is causing it, but we need to figure out what and how.