On a clear moonless night, away from city lights, you can see hundreds of stars, but most of the sky is dark. Why is it dark?

Centuries ago, philosophers argued that if the universe were infinite and unchanging, with stars everywhere, the whole sky would be as bright as the sun. In every direction you look there will be the surface of a star, no matter how far away it is. Distant stars appear smaller, but there are more of them, and the two effects cancel.

Today we know why the sky is dark at night. The universe is not unchanging, but is expanding faster and faster, ever since it started with the Big Bang nearly 14 billion years ago. Nobody knows why. But physicists describe the accelerating expansion with a concept called dark energy. It fills the universe, the same everywhere, but we do not know what it is. It is as if empty space exerts a repulsive force on itself.

I talked about dark matter – really invisible matter – in a previous column. We don’t know what that is either, except that it exerts a gravitational pull on all the matter we know – stars, planets, dust and gas – and has about five times more total mass. We think it must be made of yet-to-be-discovered particles. We keep searching in our laboratories, while theoretical physicists think about what such particles might be like. It’s a big mystery, and solving mysteries is what drives science forward.

Dark energy (really, invisible energy) is an even bigger mystery, as big as the whole universe! How do we know it is there? Our biggest telescopes can see galaxies so far away that their light has taken billions of years to reach us.

About once in a hundred years in most galaxies, a small white-hot star orbiting another sucks matter from it by gravity, becomes unstable and explodes. It becomes brighter than billions of stars, a type 1a supernova, and then fades away. These supernovae should all have the same true brightness, so measuring how faint they are, astronomers calculate their distance. From the redness of their light (red shift) they can determine their speed. The shocking conclusion that the expansion of the universe is accelerating was honored with the 2011 Nobel Prize. Will it go on forever, with the sky becoming empty in a trillion years? Then it will really be dark!