Dark is the absence of light therefore it has no differentiable rate of movement relative to it's environment.
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There are two answers that are correct, if you give the proper line of reasoning.
1. Darkness is the absence of light, since it the absence of something is not a thing, it does not truly exist and has no velocity.
2. Darkness travels at exactly the same speed as light. Think about it this way. If I'm one light year away from you, and turn on a light (bright enough for you to see at that distance), you will see that light one year after I turn it on. If I turn off that light, you stop seeing that light one year after I flipped the switch off.
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-186,282.3960 miles / sec
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Darkness travels at the speed of light. More accurately, darkness does not exist by itself as a unique physical entity, but is simply the absence of light. Any time you block out most of the light – for instance, by cupping your hands together – you get darkness. In the context of talking about speeds, darkness is what you get after the light stops coming, and therefore travels at the speed of light. For instance, consider that you are in distant space, far from all light sources such as the sun, and you have on a light bulb on the nose of your space ship. The light from the light bulb is spreading out in all directions through space at the speed of light. If you briefly turn off your light bulb and then turn it back on, there is light traveling out in all directions from before you dimmed the bulb, and behind it there is light traveling in all directions from after you dimmed the bulb. But between the two spheres of light there is no light, because no light was created when the blub was briefly off. And no light means darkness. So there is a band of darkness in between the two spheres of light. Since both spheres of light are expanding outwards in all directions at the speed of light, the band of darkness between them must also be traveling at the speed of light. You can think of darkness as what you get right after the last bit of light arrives. Since the last bit of light travels at the speed of light, the state right after must also travel at the speed of light.