from distant extra-galactic systems.

The organization of our paper is as follows. The following Section \[sec:method\] illustrates the model we use to compute the gamma-ray emission resulting frtom cosmic rays (\[sec:cr\]) and from dark matter annihilation (\[sec:dm\]), and gives details on the Fermi simulation setup (\[sec:glastsim\]). Section \[sec:origin\] discusses how to study the origin of gamma rays from clusters, including our analysis of the optimal angular region, the spectra, hardness ratios, spatial extension and multi-wavelength counterparts. We present in Section \[sec:cat\] our predictions and ranking of nearby clusters and groups according to their gamma-ray emission (the complete list is provided in the Appendix). Section \[sec:concl\] gives a discussion and summary of our results, and concludes.

Methodology {#sec:method}
===========

In this Section we present our modeling of the gamma ray emission from cosmic rays (\[sec:cr\]) and from dark matter (\[sec:dm\]). For definiteness, we consider the case of the Coma cluster [@Colafrancesco:2005ji; @grcoma], but what we find applies to generic low redshift clusters. We also discuss, in Sec. \[sec:glastsim\], the Fermi simulation setup we employ in our study.

Gamma-ray Emission From Cosmic Rays {#sec:cr}
-----------------------------------

Several mechanisms leading to the acceleration of relativistic particles in the intra-cluster medium have been discussed in 