The complete (or partial) absence of light is darkness. If you've ever lost power in your house during a nighttime storm, you know what it's like to be suddenly engulfed in darkness.
There are degrees of darkness, from the complete darkness of the woods on a moonless night to the darkness of a room lit by candlelight. Darkness is open to interpretation, since it's defined as a lack of light. There's also a figurative darkness, which can represent wickedness, sadness, ignorance, or simply mystery: "The details of their adventures would remain in darkness, unless one of them wrote a book someday."