What fundamental concept, related to the Big Bang, provides the first key solution to the darkness of the night sky?

Answer

The finite age of the cosmos

Since the universe began approximately 13.8 billion years ago, light from objects farther away than the distance light could travel in that time has not yet reached us, setting a boundary on visibility.

What fundamental concept, related to the Big Bang, provides the first key solution to the darkness of the night sky?
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