why is the sky dark at night if there are so many stars?

This is a classic puzzle known as Olbers' Paradox. It seems intuitive that if the universe were infinite and filled with stars, every line of sight should eventually hit a star, making the entire sky as bright as the surface of the sun.

The sky is dark instead because of two main reasons:

1. The universe is expanding: The universe has not existed forever, and it is actively expanding. Light from the most distant stars is "stretched" by this expansion into longer wavelengths (infrared and beyond) that our eyes cannot see.
2. The universe has a finite age: Light travels at a finite speed. Because the universe is about 13.8 billion years old, light from stars beyond a certain distance has not had enough time to reach us yet.

In short, there isn't an infinite number of stars whose light has had enough time to reach our eyes, and the expansion of space dims the light from those that are very far away.