  We examine the milli-charged dark matter scenario from a string theory
perspective. In this scenario, kinetic and mass mixings of the photon with
extra U(1) bosons are claimed to give rise to small electric charges, carried
by dark matter particles, whose values are determined by continuous parameters
of the theory. This seems to contradict folk theorems of quantum gravity that
forbid the existence of irrational charges in theories with a single massless
gauge field. By considering the underlying structure of the U(1) mass matrix
that appears in type II string compactifications, we show that milli-charges
arise exclusively through kinetic mixing, and require the existence of at least
two exactly massless gauge bosons.
