>>2616
I actually wrote my graduation thesis on dark matter candidates
the idea of extra dimensions dark matter is that there are higher, confined dimensions. confined means they're looped, with small radius. any particle moving in that extra dimension must, because of quantum mechanics, be quantised, so it must form a standing wave in that dimension. this is called kaluza-klein theory (their original idea was to explain electromagnetism by allowing this, and making particle vortices attract/repel each other).
unexcited state of matter is just ordinary matter. it doesn't move in that extra dimension. but, you can excite particles so they start orbiting along that loop. but, because the external observer cannot observe that extra dimension (as far as he's aware, it doesn't exist at all), he sees that extra kinetic energy as additional mass. the amount of this extra mass depends on kinetic energy of this movement (plus higher order adjustments).
so, in models with kaluza-klein extra dimensions, dark matter is just excited state of ordinary matter.
and no, this isn't m-theory's extra dimensions, this is something different.