Absorption of Photons from Distant Gamma-Ray Sources
  Being the largest gravitationally bound structures in the Universe, galaxy
clusters are huge reservoirs of photons generated by the bremsstrahlung of a
hot cluster gas. We consider the absorption of high-energy photons from distant
cosmological gamma-ray sources by the bremsstrahlung of galaxy clusters. The
magnitude of this effect is the third in order of smallness after the effects
of absorption by the cosmic microwave background and absorption by the
extragalactic background light. Our calculations of the effect of absorption by
the bremsstrahlung of galaxy clusters have shown that this effect manifests
itself in the energy range 1--100 GeV and can be $\tau \sim 10^{-5}$.
