Darkness represents the sin of mankind. John 3:19 20 ESV. And this is the judgment: the light has come into the world, and people loved the darkness rather than the light because their works were evil. For everyone who does wicked things hates the light and does not come to the light, lest his works should be exposed.

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."