What happens during galactitc collisions, mergers and canniblas?

Answer 1

Not much of anything. Galaxies are almost completely empty space.

What does happen is that gravity pulls objects in different directions, and the observable structure of the galaxy will change, but few stars will actually collide.

In addition, when gas clouds collide, enough bits of gas run into each other to trigger some star formation over the millions of years the collision occurs.

Answer 2

U are talking about forces that are so great it is hard to understand even a fraction. I think at present there are 2 black holes colliding and that don't sound like much .