  Hierarchical clustering of dark matter halos is thought to describe well the
large scale structure of the universe. The baryonic component of the halos is
shock heated to the virial temperature while a small fraction of the energy
flux through the shocks may be energized through the first order Fermi process
to relativistic energy per particle. It has been proposed that the electrons
accelerated in this way may upscatter the photons of the universal microwave
background to gamma ray energies and indeed generate a diffuse background of
gamma rays that compares well to the observations. In this paper we calculate
the spectra of the particles accelerated at the merger shocks and re-evaluate
the contribution of structure formation to the extragalactic diffuse gamma ray
background (EDGRB), concluding that this contribution adds up to at most 10% of
the observed EDGRB.
