The darkness of unbelief that leads to eternal darkness. But now the Light has come to dispel the darkness. There are many aspects to this metaphor, but I want to highlight just one. That is, that light reveals the truth. It shows us what God is like and what we are like.
Light reveals the true nature of things.
If we had been here four hours ago we couldn't see what we look like. It's the same with the light of Jesus. Until we come into the Light we can't see ourselves as we really are - tainted with sin. People are just like cockroaches. In the same way, we avoid the light of Christ, because we don't want to admit we are sinful.