The reason why the sky is black at night might seem like an easy answer: As the Earth rotates it turns away from the sun's light, and the sky becomes dark.
However, if you think about the millions and millions of bright stars all around the sky at night, you might start to wonder why it isn't just as bright as it was when we were facing the sun!
Believe it or not, scientists have spent lots of time trying to answer this tricky question, and have come up with many theories. Right now, the ideas that match with our current science and knowledge have to do with the age of the universe, which is thought to be about 13.7 billion years old. Light travels pretty quickly, and is often measured in distances called "light years," which refer to the distance light travels in one year. But even though light travels quickly, the universe is a pretty big place.
Because of how enormously big the universe is, the light of any stars which are more than 13.7 billion light years away are still traveling and haven't yet reached us. Their light is out there, but it's so far away that it's not brightening our night sky!
On top of that, scientists believe the universe is constantly expanding and getting bigger, meaning some of the stars that might otherwise light up our night sky are getting further away, and their light is taking even longer to reach us!
What special things have you learned about the stars in our night sky? For example, maybe you know about constellations, galaxies, or how stars are made. Have a grown-up help you learn two new things today, and then be a star by sharing your knowledge with some friends or family members!