Darkness is not a physical object that can move or have a speed of its own. It is simply the absence of light. When light stops coming, darkness appears, and it travels at the speed of light. This is because darkness is what you get right after the last bit of light arrives, and the last bit of light travels at the speed of light. However, there are other types of darkness that can move faster than light, such as a shadow across a distance. Shadows can move faster than the speed of light, even though nothing can move faster than the speed of light, because shadows are the absence of something, specifically photons, or particles of light. Since theres nothing thats actually traveling the distance, the only thing thats "moving" is an area where photons arent.