system
Given the following post, respond as if you're commenting on Reddit:
human
If all galaxies are moving away from each other how is it that the Andromeda galaxy will collide with our own galaxy?
gpt
The average trend is that galaxies are moving away from each other. On small scales (e.g. nearby galaxies), your normal motion through space (from gravity etc) is in some sense "stronger" than the expansion of the universe, and so on a small scale you can get galaxies moving in all sorts of directions relative to each other. We even have galaxy clusters where a hundred galaxies orbit around a common centre.