I Didn’t Know That . . . : Science File / An exploration of issues and trends affecting science, medicine and the environment.

Q: Why is the night sky dark?

A: Philosophers have debated this question for hundreds of years. Common sense suggests that the night sky should be bright white because there are, essentially, an infinite number of stars spread uniformly.

The reason it is not, researchers now believe, lies in the finite age of the universe--between 12 billion and 15 billion years. Even if stars stretched away into infinity in all directions, light from stars that are farther than 15 billion light-years away has not reached us yet because of its limited speed. Those stars are effectively invisible, and the sky is dark.