The origin of the diffuse extragalactic , high-energy gamma-ray background ( EGRB ) filling the Universe remains unknown .
The spectrum of this extragalactic radiation , as measured by the Energetic Gamma Ray Experiment Telescope ( EGRET ) on-board the Compton Gamma-Ray Observatory ( CGRO ) , is well-fit by a power law across nearly four decades in energy , from 30 MeV to 100 GeV .
It has been estimated that not more than a quarter of the diffuse gamma-ray background could be due to unresolved point sources .
Recent studies have suggested that much of the diffuse background could originate from the up-scatter of cosmic microwave background ( CMB ) photons by relativistic electrons produced by shock waves in the intergalactic medium ( IGM ) during large-scale structure formation .
In this work we search for evidence of gamma-ray emission associated with galaxy clusters by cross-correlating high Galactic latitude EGRET data with Abell clusters .
Our results indicate a possible association of emission with clusters at a \geq 3 \sigma level .
For a subset of the 447 richest ( R \geq 2 ) clusters the mean surface brightness excess is 1.2 \times 10 ^ { -6 } ph cm ^ { -2 } s ^ { -1 } sr ^ { -1 } ( > 100 MeV ) , corresponding to a typical non-thermal bolometric luminosity of L _ { \gamma } \sim 1 \times 10 ^ { 44 } erg s ^ { -1 } .
Extrapolating this measurement and assuming no evolution we conservatively estimate that \sim 1 - 10 % of the EGRB could originate from clusters with z < 1 .
For this cluster population the predicted non-thermal luminosity is in excellent agreement with our measurement , suggesting that the clusters have experienced mass accretion within the last 10 ^ { 9 } yrs .
If correct , then future gamma-ray missions , such as the Gamma-ray Large Area Space Telescope ( GLAST ) should be able to directly detect nearby galaxy clusters .