I totally agree with you. Sooner or later we will find out what it is, and that 80% of matter that constitutes dark matter isn't there after all.
I always hear about dark matter when they're talking in terms of clusters of galaxies. Huge amounts of matter, immense distances. However this dark matter, four times as much as the rest of the universe, is supposed to be everywhere, have mass, but only interact through the force of gravity. However, for some reason unknown to me, the visible matter in our solar system perfectly describes how the planets orbit the sun, how the moon orbits the earth, and how hard I hit the ground when I try to fly. So where is this dark matter, all this extra gravity? Shouldn't I hit the ground a lot harder than we can explain just based on the mass of our planet?
Indeed there is probably something going on at large scales, where gravity doesn't work as it does on small scales. Or indeed as you suggest the speed of light is not as constant as we believe it is, and our observations are simply distorted because of that. It's going to be tricky to find all that out, as the scales involved are so huge. On the other hand, the moment scientists find out what gravity really is by looking at the tiniest bits like the Higgs boson, we may be able to understand how the universe works at large scales.
I'm looking forward to the first theories that really explain this gravity anomaly (which is what "dark matter' really is, as I understand it: seemingly too much gravity). It may throw our understanding of the universe upside down.