Why is the sky dark at night, even though there are countless stars in the universe?

The phenomenon known as Olbers' Paradox explains why the night sky is dark despite the universe being filled with countless stars. One of the key factors is the expansion of the universe. As the universe expands, the light from very distant stars gets redshifted into longer wavelengths, some of which fall into the microwave or radio range, making them undetectable to our eyes.

Additionally, stars have a finite lifetime. Over time, stars exhaust their nuclear fuel and either dim or go out completely. This means that not all points in space are actively emitting light at any given moment.

Combining these factors — the redshifted light from distant stars and the finite lifetimes of stars — the collective result is that the light from countless stars does not uniformly fill the entire sky with brightness, leading to the dark night sky that we observe.