# Why do galaxies collide? Reconciling dark energy and an expanding universe with the fact that some galaxies are on a collision course

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??

Thanks.

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I'm afraid you get into trouble on your first supposition: the galaxies are not uniformly distributed except on the largest scale, but I will leave a detailed answer to someone with more than a passing knowledge of structure at the cosmological scale. –  dmckee Sep 15 '11 at 18:44
I'm guessing this would also have been on topic on Astronomy, but it's fine here. –  David Z Sep 15 '11 at 18:55
Almost same question was asked and answered recently: physics.stackexchange.com/questions/14657/… –  Florin Andrei Sep 15 '11 at 19:07
Thanks for the clarification! That answers it. –  Aaron Sep 15 '11 at 19:39