how far can you see in the dark? That's how long it is.
A dark-year is the opposite of a light-year. It is the distance darkness travels in 1 year. Since darkness is colder than light, it moves slower. Just how much slower is hotly debated. Near the Crab Nebula, darkness travels at about 9.3 mph, while closer to the center of the Andromeda Galaxy, darkness travels at 680 m/s due to the presence of a supermassive black hole. --Macros