Why is the night sky not filled with light from every star?
Quick Answer
The universe is finite in age, so light from the most distant stars has not had enough time to reach us. Also, expansion stretches and dims distant light.
The Full Story
This is related to Olbers’ paradox, the question of why a night sky with countless stars is dark. The answer is that the observable universe is about 13.8 billion years old, so we can only see light that has had time to travel to Earth. Because the universe is expanding, extremely distant light is redshifted and weakened as it arrives. In addition, stars are not uniformly distributed forever in a static universe; galaxies and star formation have finite histories. These facts together explain why the sky is dark at night instead of blazing white.
Key Facts
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