This seems to be a very easy question and almost everybody thinks that the answer for this question is very simple that the sky appears dark in night because earthâ€™s rotating around the sun. The part of earth which moves away from the sun will not receive the sunlight and hence sky appears dark. But this is not the reason. Answer to this question is not as simple as we think. If light of sun matters then what about the other suns that are so far from us and appear as a small star during night. Our galaxy milky way has nearly 200 billion stars and the universe has approximately 100 billion galaxies so just imagine how many suns our universe has. There are countless stars in the universe and could light up the night as the day.
Astronomers before 20th century thought that our universe is finite and it is not possible to count the stars. By this theory we could say that wherever we look in the night we must see a star and they can overlap one another just like we are standing in the middle of thick dense forest and we can see only high tress and some traces of sky. But if it was like this then the whole sky would have been shinning with light. Astronomers themselves felt problem with that statement and it was known asâ€ Olbers Paradoxâ€, a statement which disagrees with itself is called â€œparadoxâ€. Now to deal with this problem scientists gave another theory that the clouds of dust absorbs a lot of light coming from the stars. But later on they realized that if dust clouds absorb the starlight then they themselves would have been glowing so bright.
Then astronomers thought that the universe is finite and have the billions and trillions of stars which can explain why the sky is dark at night. It started approximately 15 billion years back. This means we are able to see only the stars that are up to 15 billion light years away from us. There might be more stars in the universe but we cannot see them because their light has not still arrived to earth and stars that we can see are very few to light up the whole sky. Moreover the universe is in a continuous process of expansion so the other galaxies are moving away from us and light from that stars had not still reached the earth and the stars that are too far their light will be less bright.