No no no no no. Darkness is ALWAYS THERE. It is STATIC. It is only covered up by light. So it doesn't leave and come back. What are shadows? Places in which light doesn't touch because of obstruction, a place of darkness. The darkness is always there but concealed whrn light is present. Darkness cannot enter because it is always there. When light leaves, you only are measuring the speed of the light. It is just like how you cannot measure the level of darkness. Absolute darkness is defined as the abscence of all light. It is the amount of light measured, not the amount of darkness. Once you remove all light, it can't get darker, however you can always add more light. This is a pretty basic concept. Darness doesn't move, it is always there, which means that it has no speed.