A thought experiment suggesting that if the universe were homogeneous, infinite, and unchanging, the entire night sky would be as bright as the surface of the Sun.
Most simply stated the question of "Why is the [night] sky dark? If the number of stars is infinite.." This is no longer a paradox since it is accepted today that the Universe had a finite beginning in time.
If the universe is infinitely old and uniformly filled with stars, then the night sky should shine brightly from all directions. It is, however, dark at night.