  We model and constrain the unique indirect detection signature produced by
dark matter particles that annihilate through a $U(1)$ gauge symmetry into dark
photons that subsequently decay into three-photon final states. We focus on
scenarios where the dark photon is long-lived, and show that $\gamma$-ray
probes of celestial objects can set strong constraints on the dark
matter/baryon scattering cross section that in many cases surpass the power of
current direct detection constraints, and in some cases even peer into the
neutrino fog.
