Why is the night sky dark?

When the Sun sets, the sky turns dark, scattered with points of starlight. This seems completely ordinary. But it actually hides a puzzle that troubled astronomers for centuries. If the universe is enormous and filled with a vast number of stars stretching in every direction, then why is the sky between the stars black? Should not the combined glow of all those countless stars light up the entire night sky?

The reasoning behind the puzzle goes like this. Imagine the universe were infinitely large, unchanging, and filled everywhere with stars. Then no matter which direction you looked, your line of sight would eventually land on the surface of some star, near or impossibly far. If that were true, every point in the sky would shine, and the whole night sky would blaze with light. This contradiction, between that expectation and the dark sky we actually see, is a famous problem in astronomy.

The key to solving the puzzle is that the universe is not infinitely old. It had a beginning, a finite number of years ago. This matters because light does not travel instantly; it takes time to cross space. Light from very distant stars and galaxies has been traveling toward us for a long time, but for the most distant ones, there simply has not been enough time since the universe began for their light to reach Earth yet. We can only see the light that has had time to arrive.

There is a second important reason. The universe is expanding, with distant galaxies moving away from us. As space itself stretches, the light traveling across it gets stretched too. Light from extremely distant objects is stretched so much that it shifts out of the range our eyes can see, becoming invisible forms of light. So between the finite age of the universe and this stretching of light, the night sky stays dark. That darkness is not empty or boring; it is actually a clue about the nature and history of the whole universe.

This article was written using information from Wikipedia.