Why is the night sky dark if there are billions of stars?
Quick Answer
Known as Olbers’ paradox, the sky is dark because the universe has a finite age, many stars are too far away for their light to have reached us, and expansion redshifts distant light.
The Full Story
In an infinite, eternal, static universe, every line of sight would eventually hit a star, so the sky should be as bright as the surface of a star in all directions. The fact that it is dark is called Olbers’ paradox. The main resolution is that the universe is about 13.8 billion years old, so light from very distant stars has not had time to reach us. Additionally, the expansion of the universe stretches light from the farthest sources into infrared and microwave wavelengths invisible to our eyes. Dust blocks some starlight too, but by itself that is not enough: the dust would eventually heat up and glow.
Key Facts
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Suggested video title for this topic:
"Why the night sky is dark — Olbers’ paradox explained"