I have to disagree, the Speed of Dark is slooooooooooooooooooow.
I base this on an incidence when I was in um "involved" with a girlfriend when my mom walked in. I hit the light switch but it took forever for the darkness to appear.
Adds a new dimension to the conversation. Whereas Light is affected by Gravity, is Dark affected by Human Interaction? For instance, there are more car crashes after dark. More people get drunk after dark. More people have sex after Dark.
So Dark is probably relative based on Human Interaction.
The parts of the Universe that are Dark, are only Dark when viewed by a Human Being. Before that, they were neither dark no light. Kind of like Heisenberg's Uncertainty principle, but for Dark.
I have no idea where I'm going with this.