Why is the sky dark at night? What are you talking about Carl, there is no sun of course it's dark. Well I got to thinking about this after looking at some star fields one night. There are billions upon billions of stars in every direction you look, so shouldn't their combined output leave the sky awash in light? Everywhere you look there should be a star right? Seemed reasonable so it was time for some searching.

Here is a picture explaining the question:

It seems Johannes Kepler pondered this very idea back in the early 1600's. But it really took root in the mind of Heinrich Wilhelm Olbers in 1823 and then when his work was published in 1826 it became known as Olbers' Paradox. If we assume that the universe contains an infinite amount of stars then, every sight line should end at a star, and each point in space should be as bright as a star.

The answer, it turns out, is twofold, 1) that because the universe is expanding the light from distant stars is red-shifted to be unseen and 2) because the universe is of a finite age (14 billion years) light from objects further than 14 billion light years away have not had time to reach us.

Edgar Alan Poe seems to be the first one to get it right,

"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." From 'Edgar Allan Poe: Eureka–A Prose Poem'

There you have it.