It's like train passing through the tunnel, where speed of train represents speed of light and leaving the tunnel behind for another train to come. Therefore, I would say darkness is a stationary trail on which light can travel. Also, I guess 100percent devoid of light  means less traffic (something like: the more darkish the trail, gives the best chance for visibility or flow of light(Particular beam).
Since I am not Physics major I was just curious if there is something like frictional force  between energy and stationary body. Like collision of electron slows down the reaction ultimately by transforming into different form of energy is there similar phenomenon in light energy?