system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:If galaxies are moving away from each other at tremendous speed, why are the Andromeda galaxy and Milky Way about to collide?
gpt
Dark Energy is expanding the universe, but the rate it expands is greater the longer the distance is.  
On the universal scale, Andromeda and the Milky Way are quite close together. There isn't enough distance for Dark Energy to overcome the  force of gravity for now.  
Scientist estimate that every second, each megaparsec (~3million light years) grows about 46 miles. Andromeda is .78 megaparsecs away.  Gravity is pulling out two galaxies together at 68 mi/s. As Andromeda gets closer the speed of approach will increase while the rate of expansion will decrease (because the distance is decreasing).