To be under wrath, the fruit of darkness. To be near to hell, the place of darkness. So, this is what it means to be in darkness. This darkness is manifested in the lives and behavior of those who live in it. From these two traits, it follows that those who are in darkness serve the world, and serve themselves, and serve their lusts, but they do not serve God.
Men resist the light because they love their sin.
This is the overwhelming reason that men reject the Gospel message. Faith and unbelief are bound up with light and darkness. The light shines in the darkness, but the darkness has not understood it. Shine a light in a dark room and it is no longer dark anymore. They choose to remain in the darkness though they are bathed in the light of Christ.