Why is the night sky dark? After all, if the universe is filled with billions of galaxies, every one of them swirling with billions of stars that have been emitting photons of light for billions of years, why would the universe not be awash with light? German astronomer Wilhelm Olbers pondered that question in the 1820s, and the riddle became known as Olbers's paradox. By then, astronomers and philosophers had wondered for centuries why the sky was dark and what the darkness implied about the nature of the universe. It turns out that these scholars were on to something truly profound.