The fact that the sky is dark at night shows that the observable universe cannot extend forever.

Question

Quizplus · Accepted Answer

The darkness of the sky at night is due to the fact that not all parts of the universe emit light at all times. If the observable universe extended infinitely, then every line of sight would eventually meet a luminous object emitting light, causing the sky to be uniformly bright at all times. The fact that the night sky is not uniformly bright indicates that the observable universe has a finite size, beyond which there are no luminous objects emitting light.

The Fact That the Sky Is Dark at Night Shows