system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:What space exists between galaxies and how does that differ from space inside a galaxy?

Is the space between galaxies the same as the space within a galaxy? How do they differ? And if they are similar, how do we tell where the galaxy "ends"?
gpt
Space is the same everywhere. Galaxies are just collections of matter that's all close enough to somewhat hold onto each other via gravity. The galaxy ends  where there's no more big objects like stars and such. There's always atoms and things in space so you can define a boundary in many ways, though.