It sounds like a trick question, but it troubled astronomers for centuries: if the universe is infinite, eternal, and uniformly filled with stars, then wherever you look, your gaze should eventually land on a star. The whole night sky should glow as bright as the surface of the Sun. It does not. The darkness above us is a clue, and following it leads to the biggest discovery in cosmology.

Imagine standing in an endless forest where every direction eventually has a tree in it. No matter where you look, a tree blocks the view. The same logic applies to stars in an infinite, static universe. Even though distant stars are dimmer, there are more of them at greater distances, and the two effects exactly cancel out. Every point in the sky should be covered by a star's surface, and the night should be blindingly bright.

The paradox is named after Heinrich Olbers, who popularised it in the 1820s, though others had noticed it earlier. For a long time it sat unanswered, because the assumptions behind it seemed obvious: the universe is infinite, it has existed forever, and stars are spread roughly evenly through it. One of those assumptions had to be wrong.