Until the 20th century, astronomers were stuck on a question that seems as if it should have an easy answer: Why is the night sky dark? If the infinite universe has an infinite numbers of stars, as they assumed, our evening view should be awash in their glow. Astronomers eventually got the answer to this question, known as Olbers’ paradox, when they worked out that the universe doesn’t go on forever. Our finite universe, even with its trillions and trillions of stars, doesn’t have enough stars to flood the night sky with light. On top of that, the universe is rapidly expanding—in fact, has been expanding since it first emerged—and stars were zooming away from one another and disappearing further into the darkness.
It was Edgar Allan Poe, of all people, who described our stellar surroundings best, in “Eureka,” an aptly named essay in 1848:
Were the succession of stars endless, then the background of the sky would present us a uniform luminosity, like that displayed by the galaxy—since there could be absolutely no point, in all that background, at which would not exist a star. The only mode, therefore, in which, under such a state of affairs, we could comprehend the voids which our telescopes find in innumerable directions, would be by supposing the distance of the invisible background so immense that no ray from it has yet been able to reach us at all.
The starlight that does manage to get to Earth is rather minimal. Astronomers estimate that the light is equivalent to a 60-watt lightbulb—the kind used in household light fixtures—as seen from about 2.5 miles away, in complete darkness.