The Forest of Stars
Imagine you are standing in a forest so thick that no matter which way you look, you see a tree trunk. If the universe is infinite and full of trillions of stars, shouldn't the sky be a solid wall of starlight?
This mystery is known as 'Olbers' Paradox.' For hundreds of years, astronomers wondered why the night sky is mostly black instead of being as bright as the surface of the Sun. The answer involves the very history of the universe.
A Beginning to Time
The first reason the sky is dark is that the universe isn't infinitely old. It began about 13.8 billion years ago. Because the universe has a 'birthday,' it means light from the farthest stars hasn't had enough time to reach us yet.
Light is fast, but the universe is even bigger. We can only see the stars whose light has reached Earth. Beyond that is a 'dark zone' where the starlight is still traveling toward us, like a letter that is still in the mail.
The Expanding Balloon
The second reason is that the universe is constantly expanding. Imagine drawing dots on a balloon and then blowing it up. The dots move further apart. As space stretches, it also stretches the light waves traveling through it.
This is called 'Redshift.' When light waves stretch, they change color from blue to red, and eventually, they stretch so much they become invisible 'Infrared' or 'Microwave' light that our human eyes simply cannot see.