  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.
