Why Is the Night Sky Dark?

Look up at the night sky on a clear night. What do you see?

Stars — yes. But mostly darkness.

Now here’s the problem: if the universe is infinite, and stars are spread throughout it in all directions, then every single line of sight from your eye should eventually hit a star. The sky should be as bright as the surface of the sun. Uniformly, blindingly bright. No darkness anywhere.

It isn’t. So something is wrong with that logic.

This problem has a name: Olbers’ Paradox. It was named after the German astronomer Heinrich Wilhelm Olbers, who wrote about it in 1823 — though the question is much older. Kepler raised it in 1610. Halley discussed it in the 1700s.

For over two centuries, nobody had a satisfying answer.

The First Wrong Answers

Early attempts at a solution focused on absorption — maybe clouds of gas or dust between the stars were blocking the light. But this doesn’t work. In an infinite, eternal universe, those clouds would absorb enough radiation to eventually heat up and glow just as bright as the stars themselves.

Another attempt: maybe stars aren’t distributed uniformly. Maybe they cluster, and the clusters get more sparse as you look farther out. But the math doesn’t save you. Even with clustering, the total light should add up to the same blinding brightness over infinite distances.

The problem kept surviving every proposed solution.

The Real Answer — Two Parts

The modern explanation has two components, and both matter.

Part 1: The universe isn’t infinitely old.

This is the key. Even if the universe is infinite in size, it’s finite in age — about 13.8 billion years old. Light travels at a fixed speed. That means we can only receive light from stars close enough that their light has had time to reach us.

There’s an observable universe — a sphere about 93 billion light-years across, centered on us, beyond which we simply can’t see. Stars beyond that boundary haven’t had time to send us their light yet. The sky is dark in those directions not because there’s nothing there, but because the information hasn’t arrived.

Part 2: The universe is expanding.

As the universe expands, distant galaxies are moving away from us. This redshifts their light — stretches the wavelength, reducing its energy, pushing it toward the infrared and eventually the radio spectrum, where our eyes can’t detect it. Even the light that has had time to reach us from very distant sources arrives weakened, stretched beyond visibility.

Together: finite age limits what we can see, and expansion dims what we do receive.

The darkness isn’t absence. It’s the edge of our information horizon.

Why This Matters More Than It Seems

Olbers’ Paradox looks like a quirky historical footnote — a strange puzzle about why the sky is dark at night. But solving it required confronting a much larger question: is the universe infinite and eternal, or does it have a beginning?

For centuries, most scientists assumed the universe had always existed. The paradox was one of the first cracks in that assumption. You can’t have a dark sky and an infinite eternal universe at the same time. Something has to give.

What gave was the infinite eternal part. The universe has an age. It started somewhere. And if it started somewhere, it might end somewhere too.

All of that, nested inside the question of why it’s dark when you look up.

Next time you’re outside at night, look at the dark space between the stars. That darkness is information — it’s telling you the universe is young enough, and expanding fast enough, that most of its light hasn’t reached you yet. You’re looking at the edge of everything we can know.