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boundegar t1_jd81ldo wrote

But... dark matter also doesn't interact with normal matter, so how would it create friction?


luckybipedal t1_jd8vyjo wrote

They talk about dynamical friction, which is a result of gravitational interactions. It's new to me too. I found this high level explanation:


magnamed t1_jd9s85y wrote

Just wanted to shoot you a thanks for the link.


the_funambule t1_jda2h67 wrote

you had me in the first half ngl


magnamed t1_jda6fxh wrote

Yeah I guess that doesn't really come across all that well over text. Didn't mean anything bad by it.


75MillionYearsAgo t1_jd8huam wrote

No idea. Universe is wierd as hell though so i’m sure its possible. I mean we don’t even know why gravity happens, we just know how it works. Maybe smth similar with the friction thing.


Mighty-Lobster t1_jdat4zw wrote

>But... dark matter also doesn't interact with normal matter, so how would it create friction?

Dark matter interacts via gravity. Dynamical friction is a byproduct of gravity. When you have a large massive body inside a sea of much smaller particles, the large body's gravity changes the orbits of the particles near it in a way that creates an overdensity of small particles behind the large object. That overdensity creates a gravitational pull from those particles, in the direction opposite to the body's motion. Therefore, it behaves similar to a friction force.

Dynamical friction happens in any N-body system with a sea of "small" particles and some large particles. For example, it is the reason why supermassive black holes have to always be at the center of their galaxy. Dynamical friction with the stars would bring them in.


lilrabbitfoofoo t1_jdawhvs wrote

The same results can be achieved by just putting in an appropriate amount of cold normal matter (dust, rocks, planetoids, etc.) into the same functions. We still wouldn't be able to see any of this additional matter and so it would be "invisible" to the EM spectrum (meaning invisible to us).

This provides some evidence of matter being there, not its exotic form or imaginary nature as proposed by "dark matter" theories.


popthestacks t1_jdvwru3 wrote

How do we know the slowing of these stars is caused by dark matter as opposed to the effects of time dilation as a result of a decaying orbit around a black hole?

u/Andromeda321 any thoughts on this one?