I see darkmatter as the unseen filler. There has to be "something" in between atoms, if there was nothing.... well I really don't know what would happen, but I doubt the atom would win the fight.
False reasoning. There does not 'have' to be anything. By what we know about the Universe at that scale, there is no reason why there should be anything between atoms except the forces keeping them seperate. Same goes for the internal structure of the atom; between the nucleus and the electron shells, there is (AFAWK) nothing except the carrier particles of the various forces that enforce that gap.
Dark matter is likely just another type of fermion with properties (spin, etc) that prohibit it from interacting with any particle other than the gravity carrier particle (assuming it exists). If gravity is not a carrier-mediated force, the definition of dark matter is simpler: it does not interact with any particle, but possesses mass.
As for Superstring being useless: not so. At present the predictions are untestable, but the model fits our current data (IIRC) making it at least as good as the standard model. Furthermore, the theory has shattered some preconceived ideas about the workings of the Universe; preconceived ideas when trying to find a Theory of Everything are not a good thing.