There are two view points.
1.
Darkness does not move, it is always present, and light is layed on top of it. (As an analogy, consider a table as being darkness, a tablecloth as being light: when you pull the tablecloth off, the table itself (darkness) doesn't move).
2.
Darkness moves at the same speed as light, and it meets the laws of motion in that Darkness is the equal and opposite reaction for the action of the movement of light.