  An interesting possibility for dark matter is a scalar particle of mass of
order 10 MeV-1 GeV, interacting with a U(1) gauge boson (dark photon) which
mixes with the photon. We present a simple and natural model realizing this
possibility. The dark matter arises as a composite pseudo Nambu-Goldstone boson
(dark pion) in a non-Abelian gauge sector, which also gives a mass to the dark
photon. For a fixed non-Abelian gauge group, SU(N), and a U(1) charge of the
constituent dark quarks, the model has only three free parameters: the
dynamical scale of the non-Abelian gauge theory, the gauge coupling of the dark
photon, and the mixing parameter between the dark and standard model photons.
In particular, the gauge symmetry of the model does not allow any mass term for
the dark quarks, and stability of the dark pion is understood as a result of an
accidental global symmetry. The model has a significant parameter space in
which thermal relic dark pions comprise all of the dark matter, consistently
with all experimental and cosmological constraints. In a corner of the
parameter space, the discrepancy of the muon g-2 between experiments and the
standard model prediction can also be ameliorated due to a loop contribution of
the dark photon. Smoking-gun signatures of the model include a monophoton
signal from the e+ e- collision into a photon and a "dark rho meson."
Observation of two processes in e+ e- collision, the mode into the dark photon
and that into the dark rho meson, would provide strong evidence for the model.
