Great question! The math behind this is complex but the concept is actually straightforward once you see it the right way. Forget what you learned in school for a moment and think about it from first principles.

I teach high school science and I get this question every year. The textbook explanation is a simplification — a useful one, but a simplification. The real answer involves stuff you'd cover in a college course, but I can give you the gist.

Carl Sagan's 'Cosmos' explains stuff like this better than any textbook. If you can find the old TV series or the book, watch or read it. He had a gift for making the universe make sense without dumbing it down. Highly recommend.

Don't feel bad for not getting this right away. Some of the smartest people in history spent their whole lives on questions like this. The fact that you're curious enough to ask puts you ahead of most people. Keep asking why.

I'm a physics grad student and this is one of my favorite questions to explain. The key insight is that our everyday intuition doesn't always apply at extreme scales — very small (quantum), very large (cosmological), or very fast (relativistic).