ELI5: Why is the sky dark at night?

Imagine the Sun is like a big lightbulb, shining light everywhere. During the day, the Earth is facing the Sun, so we get all that light. That's why it's bright!

Now, think of the universe as a giant room. And imagine that room is filled with billions of stars, like tiny lightbulbs spread out everywhere. So, if you look in any direction, you should see a star, right? And if you see a star, shouldn't the whole sky be bright, even at night?

That's a good question! It's called Olbers' Paradox.

The answer has a few parts:

• The Universe is Expanding: Imagine blowing up a balloon. As the balloon gets bigger, the dots you drew on it get farther apart. The universe is doing something similar! The stars and galaxies are moving away from each other. This means the light from very far away stars gets stretched out, like pulling on a rubber band. When light stretches, it becomes redshifted, which means it loses energy and becomes dimmer. Some light gets stretched so much we can't even see it anymore!

• The Universe is Not Infinitely Old: The universe had a beginning (the Big Bang). Light from stars that are very, very far away hasn't had enough time to reach us yet. It's like waiting for a letter to arrive – if it's sent from really, really far away, it might take a long time! Since the universe hasn't been around forever, the light from some stars just hasn't made it to Earth yet.

• Dust and Gas: There's stuff floating around in space, like dust and gas clouds. This stuff blocks some of the light from distant stars. It's like fog on a road – you can't see very far because the fog gets in the way.