>In some cases dark matter has been separated from the galaxy it was associated

Isnt the bullet cluster famous for this?

How do we even know that?

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

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. :(

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. :(

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. :)

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.

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.

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.

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.