speed of dark? are you saying the inverse of C? where the light arrives to the point of dark I guess?
I work for a florescent dark sucker company. See, our lamps glow brightly because they soak up all the surrounding dark. A battery is a form of a dark sucker too. It’s sucks in dark until it’s full of dark and then you have to recharge it by pumping out all the dark. I know it’s confusing but physics is not for the timid. You should always seek to go boldly where no man has gone before...hmmm That sounds familiar.
Beam me up Scotty!