Interstellar Medium

When you look up at night, you see a bunch of little bright points in the sky. They're called stars, and they're enormous balls of glowing plasma/gas that are tremendously far away. But you knew all that. What you might not have known is that there's also a ton of stuff between us and them. It's called the interstellar medium (actually, that's mostly within galaxies. Between galaxies is the intergalactic medium, and it's even less dense). It's made of incredibly diffuse molecules - on average, there's only around one to three molecules per cubic meter. Think about that for a second. At standard temperature and pressure (pretty close to the room you're in right now, probably), there are around $10^{28}$ molecules per cubic meter. So the interstellar medium doesn't really have much stuff in it. In fact, it's a better vacuum than any that we can create here on Earth. This opens up all sorts of interesting lines of inquiry, like how chemistry works when molecular collisions are so rare. Physics is cool.