The theme of light and darkness is sprinkled throughout scripture.
Light usually represents divine revelation - some kind of cut-through insight or transition in understanding. Light represents being able to see things more clearly. Light means that when was previously obscured is now out in the open.
Darkness, by contrast, represents the more natural state of people. Darkness means not being able to see what is right in front of us. Darkness can hide things even when they are out in the open!
Darkness is the absence of light. When light comes, darkness is done away with.