My layman understanding of cosmology is:
- galaxies are uniformly (more or less) spread throughout the universe, per the Big Bang and the fact that in a central explosion, all dispersed points are racing away from one another
- dark energy is increasing this dispersal, by accelerating galaxies further apart.
My confusion is, reconciling the above two facts with the occasional statement that galaxy X and galaxy Y are on a collision course (e.g. our Milky Way and Andromeda). Or statements about two galaxies whose collisions were just observed.
I understand that the collisions are driven by gravity, naturally.
What I don't understand is... are these collisions exceedingly rare? A common occurrence? If galaxies are mostly uniformly dispersed throughout the universe, and are accelerating away from one another in all directions, then shouldn't the number of collisions be decreasing over time? Approaching zero??