Why is the night sky dark even though the universe has so many stars?

This puzzle is known as Olbers’ paradox, and the modern answer comes from cosmology. The universe is about 13.8 billion years old, so we can only see light that has had time to travel to Earth. Many stars and galaxies are too far away for their light to reach us yet, and some light is shifted into longer, fainter wavelengths by cosmic expansion. Dust alone cannot explain the darkness, because dust would eventually heat up and glow. So the night sky is dark mainly because the universe is not infinitely old, and it is expanding.