1:5

1:5 And the light shines in the darkness, and the darkness did not overcome it.

In the first creation, darkness was upon the face of the deep (Gen. 1:2) until God called light into being, so with the new creation there is spiritual darkness on the face of the human heart until the the light of men which is the life in Jesus shines upon it.

Apart from the light in Christ the world is encompassed by darkness. The Greek word katelaben is translated overcome in the RSV. According to Bruce mastered would be another good translation for katelaben. We know that light and darkness are opposites, but they are not opposites of equal power. Light is always stronger than darkness and darkness can not overcome light or master it. One small candle can illuminate a dark room and not be dimmed by the darkness there.

We often here darkness and light taught in an equal type of dualism. The Bible dispels this by showing us that the true light Jesus has come into the world and darkness is being overtaken.

Sources: The Gospel of John, F.F. Bruce