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You already answered your question.....but I'm gonna add somethin'
its because of the absence of light. It also has something to do with our eyes. Our eyes can see COLORS because the LIGHT RAYS are bouncing off the objects, and because of that, we can see an objects color.
*I hope this somehow helped you....*
peace out bro!
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The absence of light results to also to the absence of the light colors, common example is white. Well, if there is no "light color" or simply white, there will be a "dark color" or simply black. 

Let us say we are in a dark room, light is absent on that room. Now what color do we see? Yes, it is black. Because LIGHT is commonly WHITE, and in the contrast of light, DARK is commonly BLACK, and without WHITE there is BLACK, and without BLACK there is WHITE.