  We discuss possible implications of a large interaction cross section between
cosmic rays and dark matter particles due to new physics at the TeV scale. In
particular, in models with extra dimensions and a low fundamental scale of
gravity the cross section grows very fast at transplanckian energies. We argue
that the knee observed in the cosmic ray flux could be caused by such
interactions. We show that this hypothesis implies a well defined flux of
secondary gamma rays that seems consistent with MILAGRO observations.
