Why is the night sky dark even though there are billions of stars?

If the universe were infinite, eternal, and filled uniformly with stars, the whole sky would be bright all the time, a puzzle known as Olbers’ paradox. But the universe has a finite age of about 13.8 billion years, so light from extremely distant objects has not had enough time to reach us. In addition, the universe is expanding, which stretches light to longer, dimmer wavelengths through redshift. Dust can absorb some light too, but the main reasons are cosmic age and expansion. So even though there are billions of stars and galaxies, most of their light is too far away, too stretched, or has not arrived yet.