Why is the night sky dark? It's a bigger clue than it looks

Here's a question that sounds childish but stumped brilliant minds for centuries: why is the night sky dark? If the universe were infinite, eternal, and evenly filled with stars, then every single line of sight — no matter which direction you looked — would eventually land on the surface of a star. The whole sky should blaze as bright as the Sun. This puzzle is called Olbers' paradox.

The resolution is stranger and more beautiful than the puzzle. The universe is not infinitely old — it began about 13.8 billion years ago. Because light travels at a finite speed, we can only see stars whose light has had time to reach us. Beyond that horizon, there simply hasn't been enough time for the glow to arrive. The darkness between the stars is, in a real sense, a snapshot of the universe's youth.

There's a second reason: the universe is expanding, stretching the light from the most distant objects. Light that started as visible glow gets redshifted into invisible infrared and microwaves. That faint leftover heat still fills the whole sky — we call it the cosmic microwave background. So the sky isn't truly empty after all; the ancient blaze is there, just shifted out of sight.

In other words, the plain darkness overhead is quiet evidence that the cosmos had a beginning and is still growing.