re: Colliding galaxies reveal dark matter
The stars are so far apart, even in the densest parts of galaxies, that stellar collisions are next to impossible. But orbits are disrupted and stars are scattered around all over the place, in "tails", and many are even ejected altogether.
However dust and gas clouds DO collide, causing bursts of star formation along shock fronts and supernovae as the most massive of the new stars burn out and rush through their evolution. Galaxies are swept clean of dust and gas and stellar populations disrupted.
If the process didn't happen so slowly, it would be pretty spectacular.