Darkness, being an absence of electromagnetic waves (light) has nothing to do any waving. The somewhat longer answer (that sound like it undermines the short answer, but totally doesn’t) is that “darkness”, while not a wave itself, can be produced by light waves. Light waves, rather than merely being there or not can add and subtract from each other (at each tiny point in space). So if you’re looking at a particular point in space there can be light waves present, but they may cancel each other out, creating a little dark patch. Long story short: darkness isn’t a wave itself because it isn’t anything, if it has a speed it’s the same as light speed, and also light is profoundly weird stuff (and not to be taken lightly).