The sky at night is black because there is no light in space to reflect off of. During the day, the sun gives off light that illuminates the sky and makes it appear blue. But at night, the sun is on the other side of the earth and its light is not reaching the sky where we are.
Also, the stars and planets that we see at night are very far away from Earth, meaning that their light is too weak to illuminate the sky. The light from these stars and planets is not enough to counteract the darkness caused by the lack of sunlight.
Additionally, the atmosphere around Earth absorbs some of the light from the stars and planets, making them appear dimmer. This is called airglow, and it makes the sky appear even darker.
So, the next time you look up at the night sky and see it as black, remember that it is because there is no light source nearby and the atmosphere absorbs the light from the stars and planets making them dim.