system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is the universe like outside of the boundaries of galaxies? Is it just empty space?
gpt
Intergalactic space (the space between galaxies) is pretty much empty, yes. In any case a lot more empty than interplanetary space. It is not strictly "emtpy" as such, and not going to discuss if something like "nothing" even exists, but short of the occasional rogue star that got kicked out of its galaxy, particly density is very very low there. Galaxies string up a bit like soap foam: they accumulate along a bubble edge while the bubbles itself basically consist of emptiness inside.

EXCEPT, and this is a big except, there are huge amounts of Dark Matter and Dark Energy out there, much more than, "normal" matter, which play a vital role in galaxy, cluster and supercluster formation (i.e. galaxy clusters and clusters of galaxy clusters). Unfortunately we have no real clue what either of them are, except that we can't detect them (hence "dark") and that they must be quite different from the stuff that e.g. you and me are made of (i.e. baryonic matter).

Oh yeah, and besides "cold", it's also a heck of a lot darker out there than e.g. your typical starry night sky is. The stars you see at night when you look up are mostly in our more or less immediate neighbourhood from a few dozen to within a few thousand lightyears at best. Our own galactic center tends to be barely visible, our own neighbouring galaxies are so faint to us at their distance that most people have probably never noticed them. Now imagine you'd be situated in an intergalactic bubble where there is nothing but emptiness around you and the next visible galaxies are millions of lightyears away. Not much to see there except for some oddly faint smudges.