Poor phrasing on my part, I think. Brane models (whether in M-theory or elsewhere) are certainly one way of producing this sort of phenomenon theoretically. (I'

Poor phrasing on my part, I think. Brane models (whether in M-theory or elsewhere) are certainly one way of producing this sort of phenomenon theoretically. (I've done research on branes in both string and M-theory.) But ultimately, what you've described is just as arbitrary as declaring "this quantum field has no electric or color charge": there's no reason known that we'd happen to have those two brane stacks in the right positions to mimic ordinary matter interacting with dark matter. (Why exactly the right branes and intersections to reproduce the standard model? Why also this separate brane or branes at a carefully chosen distance to have meaningful gravitational interactions but not gauge interactions? What stabilizes the separation of the branes over cosmological timescales? etc.)