Re:Correct me if I'm wrong... (Score 4, Interesting)

There are current astrophysical models which postulate that the Universe is a hyperplane embedded in higher dimensional space, called Randall-Sundrum models. In which case gravity can propagate trasverse to this plane, hence there can be matter outside the Universe which can still interact with it. There is also the idea that this 'brane' (as it is called - nothing to do with zombies) has a small extra dimension (less than 1mm in size) so the current gravitational law needs to be changed to r^{-4} or so at very short distances.

String theory as such tends not to comment on dark matter (could be D0-particles, could be fish) as no-one knows how to compactify it down from 10D and break supersymmetry in a useful way.