Light and darkness works on counter direction. Like upthrust principle, when you put some object on water - water tries to push the object upward direction and the object tries to push the water in downward direction. Who so ever's force is higher will appear.

Like air pressure on atmosphere and blood pressure of human which balances the body. Our blood pressure inside ourself tries to oubrust but the air pressure outside keeps it normal.

In the case of light and darkness. Its like air pressure of atmosphere which is found every where and blood pressure inside us (single instance for each one of us and every living being)

. Darkness covers most of the part in universe but when the light comes out of any single instance lets say sun , stars etc it tries to covers a portion of it on the universe and replaces the darkness according to its power. Just trying to find its existance.

Its just my thought.........