If the universe is endless and full of stars, why is the night sky dark?
Imagine an infinite, unchanging universe evenly filled with stars in every direction. Looking far enough along any line of sight, you should eventually hit a star's surface, meaning every point in the sky should blaze as bright as the sun. So why, standing outside at night, do you mostly see darkness between the stars?
Reveal the answer
Because the universe isn't infinite in age or static: it's roughly 13.8 billion years old and expanding, so light from most distant stars hasn't had time to reach us yet, and the light that has arrives stretched into invisible wavelengths by the expansion. The puzzle is named for Heinrich Olbers, who wrote it up in 1823, though earlier thinkers had noticed the same problem — and its resolution became one of the first solid pieces of evidence for a universe with a beginning.