Why does the night sky appear dark?
Two scientific reasons. Explained loosely and briefly below. But, watch the video it’s a much slicker way to understand.
If we lived in a static and infinite universe filled with stars, we’d expect that we’d be able to see stars in absolutely every direction, the night sky would be full and would be as bright as the sun during the day. However, we don’t. The Universe is finite although it has no spatial “edge”. Because the Universe had a beginning, there simply aren’t stars in every direction because there was a time before stars first formed. Moreover, because the Universe is expanding, the oldest, and so most distant stars, are red-shifted (it’s not quite the Doppler effect because it’s the medium itself that’s expanding not the energy source moving away from us through that medium). So, when we peer into the most ancient depths of space with the most powerful telescopes we cannot see those stars that have been red-shifted so much that the light from them reaching us is in the infrared.