It might sound like a stupid question at first, because of all the black darkness in between the stars, but the answer is actually evidence of one of the fundamental properties of our universe. The problem comes in when you realize there shouldn’t be any darkness at night: there are trillions of trillions of stars in the universe, and their light should literally be filling up the sky all the time. So then why is it mostly dark? Some stars are really far away and their light hasn’t gotten here yet, but there are still plenty of stars whose light should be flooding our tiny planet. Which it is — we just can’t see it, because by the time the light gets here, it’s infrared.
If the universe were constant and stars stayed in the same position, the night sky would be indeed be filled with light. But because the universe is expanding, stars are constantly moving away from each other, and therefore away from us, too. That motion causes redshift, which is what happens to light from an object that is moving away: the wavelength increases due to the Doppler effect, and since color is dictated by the wavelength of light, it first appears more and more red and then infrared, which we can’t see anymore. So the sky is dark because most of the light that we would normally see — from all the countless stars — has been shifted to infrared on its way to Earth — because the universe is expanding.