Science
Why is the sky dark at night if the universe is full of stars?
- The universe has a finite age, so not all light has arrived
- Space absorbs light
- Stars switch off
The answer is The universe has a finite age, so not all light has arrived. Olbers' paradox - light from the furthest stars has not reached us.
The question is older and better than it looks, and it is called Olbers' paradox. If the universe were infinite, eternal and evenly filled with stars, then every line of sight would eventually land on a star's surface and the whole sky would blaze as bright as the Sun.
It does not, so at least one of those assumptions is wrong - and the answer is age. The universe is about 13.8 billion years old, so light from beyond a certain distance has simply not had time to arrive; and because the universe is expanding, light from the most distant sources is stretched out of the visible range entirely.
Edgar Allan Poe got close to this in 1848, in a prose poem, arguing that the void we see must mean light from the furthest reaches has not yet reached us. The sky is not truly dark either: point a radio telescope anywhere and you find the microwave background, the cooled glow of the early universe, filling the entire sky. The blaze is there, just stretched to a wavelength eyes cannot see.