  We study the possibility of detecting a signature of particle dark matter in
the spectrum of gamma-ray photons from active galactic nuclei (AGNs) resulting
from the scattering of high-energy particles in the AGN jet off of dark matter
particles. We consider particle dark matter models in the context of both
supersymmetry and universal extra-dimensions (UED), and we present the complete
lowest-order calculation for processes where a photon is emitted in dark
matter-electron and/or dark matter-proton scattering, where electrons and
protons belong to the AGN jet. We find that the process is dominated by a
resonance whose energy is dictated by the particle spectrum in the dark matter
sector (neutralino and selectron for the case of supersymmetry, Kaluza-Klein
photon and electron for UED). The resulting gamma-ray spectrum exhibits a very
characteristic spectral feature, consisting of a sharp break to a hard
power-law behavior. Although the normalization of the gamma-ray flux depends
strongly on assumptions on both the AGN jet geometry, composition and particle
spectrum as well as on the particle dark matter model and density distribution,
we show that for realistic parameters choices, and for two prominent nearby
AGNs (Centaurus A and M87), the detection of this effect is in principle
possible. Finally, we compare our predictions and results with recent gamma-ray
observations from the Fermi, H.E.S.S. and VERITAS telescopes.
