After sunset, the sky changes dark gradually. Why is the night sky dark? The darkness is came from the universe.

But there are thousands of stars in the universe, not only bright stars but also emitting gas clouds and high energy objects. However the night sky does not bright. Why?
After sunset, the sky becomes dark gradually. Why is the night sky dark? The darkness comes from the universe.

But there are thousands of stars in the universe, not only bright stars but also emitting gas clouds and high energy objects. However, the night sky is not bright. Why?
If the universe spreads out to infinity and it has been from the limitless past, you would see stars in every direction. The universe should be filled with star light. But in reality, it is not. This is known as "Olbers paradox".

Olbers' theory includes some wrong assumptions. At first, stars are not distributed equally in the universe. Stars form the galaxies, galaxies form clusters of galaxies, and clusters of galaxies form super clusters of galaxies. Since the universe has a hierarchical structure like this, the relative density of stars decreases as you see a larger scale universe. Furthermore, the lights from far stars are weaken by readshift, because the universe is expanding. Thus, the far places in the universe look dark relatively. Finally, the universe is old. But the time the universe spent is not limitless. In other words, the light from the far universe has not yet arrived here. These are the reasons why the universe is dark, now.