Question: Why is space a vacuum?

Alex Seeney answered on 22 Jun 2018:

In short, because it’s really big! Over time, gravity tends to make matter clump together, so we are left with collections of matter (i.e. planets, stars, etc) and these are surrounded by a very, very large amount of space! Douglas Adams phrased it well:

“Space is big. Really big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”

Space isn’t a complete vacuum though, as we still have some particles floating around out there.