system
Given the following post, respond as if you're commenting on Reddit:
human
I was reading about nebulae on wikipedia and saw that they are formed when the interstellar medium collapses. What exactly is the interstellar medium, and how does it collapse?
gpt
It is essentially empty space, but filled with very little amounts of hydrogen, helium and other atoms. It accretes together over long periods over time, and start collapsing on its own gravity. It also starts to spin, and then it starts to flatten. 

Space is supposedly a vacuum, but interstellar medium has a LITTLE bit of matter in it.