system
Given the following post, respond as if you're commenting on Reddit:
human
How is Milky way and Andromeda galaxy going to colloid if the universe is expanding and everything is going further apart?
gpt
There are two parts to this. The first part is that galaxies are moving around anyway relative to one another, even without the universe's expansion, but that peculiar motion of each galaxy doesn't tend to be either towards or away from us. 

The second part is that Andromeda and the Milky Way are very close together, in the scheme of things, and the expansion of space is hard to notice until you've got a LOT of space in between things. While space is expanding, it's only adding about 70 km/s per Megaparsec of distance between things. A Megaparsec is about 3,240,000 light years. Andromeda is only 2.5 million light years away, so the universe's expansion is only adding about 54 km/s or so. Galaxies in groups tend to move faster than that, though, around 100-200 km/s. Andromeda is moving towards us at 110 km/s, for example. Something 4 times further away than Andromeda would be much less likely to be moving fast enough to merge with us eventually.