Comment by potamic
15 hours ago
> If two typical galaxies met in a head-on collision, the dark matter would experience very little friction, or drag, and would keep on rocketing by. But ordinary matter, which interacts using all four fundamental forces, would get all tangled up.
Isn't a galaxy mostly empty space? I thought I read before that when galaxies collide there's actually very little physical contact between the matter. Stars mostly pass through affected predominantly only by change in surrounding gravity. Is this not the case?
The ordinary matter that gets tangled up is interstellar dust clouds, not stars and planets.
The mean free path of a hydrogen molecule in the "warm interstellar medium" is about 0.03 light years, much smaller than the diameter of a galaxy. So if gas clouds collide they will interact strongly.
They don't collide but they do get "tangled" in each other's gravity. Small things (stars) get tossed in random directions and big things (gas clouds) pass into each other and start condensing into smaller/brighter things.
It can't be in each other's gravity, else the same phenomenon would affect dark matter. For something to affect regular matter but not dark matter, it has to be an electromagnetic interaction (at this scale - as the weak and strong forces only interact on very very small distances). So it has to be a significant amount of head-on collisions, or some kind of charged dust clouds projecting large EM fields.
I interpreted that as gravity causing things from the two galaxies to mix up, after which normal matter can interact electromagnetically (in more head on collisions than usual) but dark matter won't.
Very big object x Very small chance x Very long time = normal probabilities