Why is space black if there are billions of stars?

If there are billions of stars out there, why is the night sky dark? Think about it. Every direction you look should eventually end on a star. The whole sky should be blazing with light, like daytime. It isn't.

01A puzzle from 1823

An astronomer called Heinrich Olbers got stuck on exactly this back in 1823. If the universe goes on forever and is filled with stars, the sky should be bright from edge to edge. The question became known as Olbers' paradox.

02Webb made it worse

NASA's Webb telescope only sharpened the problem. In 2022 it looked at a tiny black spot of sky, about the size of a grain of sand held at arm's length, and found thousands of galaxies in it. Each of those galaxies holds billions of stars.

So the light is out there. Two things hide it from us.

03First: time

The universe is about 13.8 billion years old. Light travels fast, but not infinitely fast, and light from really far away simply hasn't reached us yet. We only see stars inside the distance light has had time to cross, and that leaves gaps of darkness between them.

04Second: space is stretching

The universe is expanding, and that expansion stretches light as it travels. The oldest light of all, released long before the first stars, has been stretched so far that it now arrives as microwaves. Our eyes can't see microwaves.

05The answer Olbers got wrong

Olbers himself blamed dust. He thought clouds of dust between the stars were soaking up the light. But dust that absorbed that much light would heat up and start glowing too, so the sky would still be bright.

So the sky isn't black at all. It's glowing everywhere. Our eyes just can't see it.

That glow is called the cosmic microwave background, and it covers the whole sky. Its temperature is only about 2.7 degrees above absolute zero, roughly −270 °C.