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Cheap Astronomy
2019-May-17, 07:07 AM
As I understand it we appeal to dark matter in a halo about the Milky Way to explain why outer parts of the galaxy can maintain the orbital speeds they do without being flung outwards. But if dark matter was a kind of invisible dust wouldn't it decay the orbits of outer stars by obstructing their motion? So, what does this say about dark matter, it just has mass and generates gravity, but otherwise has no physical form? Thanks

dtilque
2019-May-17, 08:15 AM
The reason dark matter doesn't cause orbits to decay is that it doesn't interact with baryonic matter except gravitationally. Things in Low Earth Orbit, for example, interact with the tenuous outer part of the atmosphere and have their orbits decay. That's due to friction, which is an electromagnetic force. There's no friction between dark matter and regular matter.

There is one way that orbits decay due to gravity and that is tidal drag. However, at galactic scales, that's such a ridiculously small effect that it may as well not exist.

antoniseb
2019-May-17, 11:32 AM
Cheap Astronomy might be thinking that it is similar to how in young planetary systems, the large planets migrate inward by exchanging momentum with smaller objects. This effect could potentially be working in the galaxy, but very slowly, since the dark matter is mostly not in the disk of the stars, and the dark matter particles mostly don't have orbits in that plane and the total mass of dark matter particles that might exchange momentum with a star should be pretty low. If you look at the very center of the galaxy, the dark matter might be dense enough that it is an important factor, but that is me guessing, not from looking at specific computer models.