Evidence for indirect detection of dark matter from galaxy clusters in Fermi $\gamma$-Ray data

We search for spectral features in Fermi-LAT gamma-rays coming from regions corresponding to eighteen brightest nearby galaxy clusters determined by the magnitude of their signal line-of-site integrals. We observe a double peak-like excess over the diffuse power-law background at photon energies 110 GeV and 130 GeV with the global statistical significance up to 3.6\sigma, confirming independently earlier claims of the same excess from Galactic centre. Interpreting this result as a signal of dark matter annihilations to two monochromatic photon channels in galaxy cluster haloes, and fixing the annihilation cross section from the Galactic centre data, we determine the annihilation boost factor due to dark matter subhaloes from data. Our results contribute to discrimination of the dark matter annihilations from astrophysical processes and from systematic detector effects as the possible explanations to the Fermi-LAT excess.