system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How did we figure out that space is "space"?

How did we know that beyond our own atmosphere there is a void of oxygen (er... anything) without trial by error?
gpt
A couple of simple observations allows you to deduce that space is mostly empty.  One is that air tends to get "thin" as you move up a mountain... the air has less gas in it the higher you get.  The second one is that when you shine light through something, you can make some measurements of what it is made of.  Because the sun's light comes through space before it hits us *and* we know what the Earth's atmosphere is made of, we can tell that the sunlight is only really hitting our atmosphere -- thus, space must be more or less empty.

First try at one of these... hope it wasn't a bucket of failure.