Something I learned during this was that the theory of why the galaxies collide, even though they are said to be expanding away from each other, is that it is like a cosmic entanglement. Let them get a little too close, while on adjacent trajectories, and they will tend to merge/collide/crash.
Apparently the reason they do this, is because of gravity. However, there is not enough mass in the galaxies to exert such a pull. So.... dark matter. A fudge factor for explaining a process we don’t fully understand.