Seeking dark matter with γ-ray attenuation

The flux of high-energy astrophysical gamma rays is attenuated by the production of electron-positron pairs from scattering off of extragalactic background light (EBL). We use the most up-to-date information on galaxy populations to compute their contributions to the pair-production optical depth. We find that the optical depth inferred from gamma-ray measurements exceeds that expected from galaxies at the similar to 2 sigma level. If the excess is modeled as a frequency-independent re-scaling of the standard contribution to the EBL from galaxies, then an excess (an overall 14-30% increase of the EBL) is favored over the null hypothesis of no excess at the 2.7 sigma level. If the frequency dependence of the excess is instead modeled as a two-photon decay of a dark-matter axion, then the excess is favored over the null hypothesis at the 2.1 sigma confidence level. While we find no evidence for a dark-matter signal, the analysis sets the strongest current bounds on the photon-axion coupling over the 8-25 eV mass range. This work highlights the sensitivity of gamma-ray optical depth measurements to ALPs, which is expected to improve with new observatories and better EBL determinations from future observations.