Why is the sky black at night?

Many are those who wonder why the sky is dark at night, the answer lies in the age of the universe. The universe is 15 billion years old, not enough time for light to travel to us from stars and objects at distances greater than 12 or 13 billion years.

So the universe is not infinitely old, and infinitely extended, nor the stars shine forever. If all this is given, the sky would be as bright as the sun's surface, and thus would be day and night. That's because every place in the universe would be covered by a star and light would fill everything.

But as we see the universe has a finite age, so there is less light to fill all space. And if that were the stars die, so could not be that way. And we also lose ability to see them from here because of the very expansion of the universe that makes the light radiated from distant stars we arrive.

Source: AstroUcla