The speed of light... as darkness is the absence of light, then its speed would be the same, for something becomes dark as the light moves away from it.
Well, actually there is no darkness. It is not a substance or energy. It is the absence of light. It has no particles or waves in order to measure its speed or mass or volume because it has none. Just like cold is the absence of heat (we measure heat), void is the absence of space (we measure space). In fact darkness is simply a term to define the absence of light. So, it doesn't really exist. Without light we cannot see so that black nothingness, the absence of light we name "darkness". Advanced stuff. lol
Just one nitpick though, we don't measure heat, we measure the activity of molecules, which we call heat. The difference between heat and cold is only a difference in the activity of molecules. Water at 40 degrees F, has less molecular activity than water at 100 degrees F. We call it hot and cold, but that is all subjective. Temperature (measured on one of the three scales is a measure of molecular activity.)