eli5:with billions of stars emitting photons why is the night sky not bright?
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As many others have said it’s because space is big, but this is actually a very important observation – if space was infinite and timeless, as many believed just over a hundred years ago, it WOULD be bright – every line of sight would end in a star. The observation that it’s dark really supports the idea that the universe isn’t infinitely old (since if it was, light from however far away would be able to get to us) and also supports its expansion (since the light can shift away from visible into the “cosmic background radiation.” So yeah, this is a great question and a very important idea!
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