Note: Consider the behavior of stars when galaxies collide, compared to the behavior of galaxies when clusters of galaxies collide.
1. Stars are denser than galaxies, on average.
2. When galaxy clusters collide, the number of galaxies involved measure dozens to thousands.
3. The distances between galaxies is comparable to their size.
4. When galaxies collide, the number of stars involved measure in the billions to hundreds of billions.
5. Galaxies are bigger than stars.
Read more:
Why are galaxies more collisional than stars?