system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: With billions of stars in our galaxy, why is the night sky so dark?

I hope this is not too stupid of a question!  Under the starry sky a few nights ago I could quite clearly see the band of the Milky Way trailing across the sky, which started me thinking: with so many stars in our galaxy, why is the night sky so dark?  Is it just because of the sheer distances involved combined with very weak light intensity?
gpt
It's not a stupid question. It was actually a very puzzling one, a long time ago when it was first thought up by a guy named Olber.

His thinking went like this: Since the universe is infinite, every possible straight like you can draw from your eyeball out into the night sky must, sooner or later, end at a star. Why, then, is the sky dark? Why isn't it a solid wall of blinding sunlight?

The answer is that it takes time for light to cross distances. While we can imagine, in principle, that every line you can draw from your eye out into space ends at a star, in truth virtually *none* of the lines you can draw from your eye out into space end at a star *within fifty billion light-years.* That's as far away as a star can be in order to be seen at all, because the light from any stars farther away from us than that hasn't yet had time to reach our eyes.

So if you imagine drawing a line from a point on the surface of the Earth out into space in some direction at random, the odds are *incredibly tiny* that that line will end at a star within fifty billion light-years … and everything beyond fifty billion light-years is invisible to us, because light from that far away hasn't reached us yet.