Week 1: Why is the night sky dark?
The darkness between the stars looks empty. It is not. A dark night sky already tells us that the universe has a history.
27 July 2026
Look outside on a clear night. Most of the sky is black.
That seems so ordinary that it hardly looks like a physics question. Stars are bright. Space between them is dark. What is there to explain?
Imagine a universe that is infinite, eternal, and filled more or less uniformly with stars. Pick any direction and follow your line of sight far enough. Eventually it should hit the surface of a star. Pick another direction and the same problem appears.
If the universe really had those properties, every patch of sky should eventually end on something bright. Distance makes an individual star look dimmer, but there are also more stars at greater distances. In the simplest version of the argument, those effects do not rescue the dark sky.
The whole sky should glow.
It does not.
So the darkness is evidence.
At least one part of that imagined universe is wrong. Our universe has a history. It has not contained shining stars forever, and light has had only a finite time to travel. The universe is also expanding, which stretches light from distant sources and changes the energy that reaches us.
The full story is more complicated than the slogan “the universe has a finite age.” Stars themselves have finite lives. Matter is not distributed with perfect uniformity on every scale. Cosmic expansion matters.
But none of that weakens the original point.
The dark between the stars is doing work. Something that looked like an empty background has already told us that the universe cannot be an eternal, static sea of stars.
And once you know the universe has a history, the obvious next question is not about darkness anymore. It is about light.
What we explained#
A permanently static, eternal universe filled uniformly with stars would not give us the night sky we actually see. The darkness carries information about cosmic history: the universe has a finite past, stars have not shone forever, and expansion changes the light travelling across it.
What became stranger#
What is the oldest light we can see?