We discuss here methods and results concerning the angular cross-correlation between gamma-ray sky-maps and catalogs of galaxies and galaxy clusters.
The main goal of the study is to extract information on the clustering, redshift distribution and type of the unresolved non-thermal gamma-ray sources. We will show how this technique can be used to search for "new" populations of sources, including galaxy clusters and the one possibly provided by annihilation or decay of particle dark matter.