So it is a known fact that the Andromeda galaxy will collide and merge with the Milky Way galaxy. In the video it shows the collision. Time 1:40 My question is how does it take only 3 collisions for the merger to take place? First all, can we assume that if no stars/dark matter/planets from one of the galaxies affects the stars/dark matter/planets from the other galaxy , the galaxies would just keep passing through each other in an infinite oscillation? My reasoning is that if there is no internal friction, then energy is conserved, if we regard the two galaxies and objects moving with a velocities associated with their respective CMs and internal disturbance as something analogous to transfer of heat for collision between classical rigid objects. Seems to make sense classically, but I think GR needs to be considered as well, and I don't know how this would affect things. Second, the oscillation seems to be heavily damped. We know the "friction" doesn't comes from actual collisions of planets and stars, as explained by the video since the distances between celestial objects are quite large. So does this mean that the cause of the "friction" or internal disturbance is due to them just falling into orbit with each other,?