system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly is in the space between galaxies?
gpt
A very, very, very small amount of hydrogen. An even smaller amount of various other elements, free electrons, etc. An average density is one hydrogen atom per cubic meter. It’s just about perfect vacuum. 

To get an idea of how close to vacuum this is, if you had a 1 liter bottle of hydrogen gas at STP, it would fill a space roughly the size of Pluto at this density.