We discuss methods and results concerning the angular cross-correlation between sky-maps of the extragalactic background radiation in different wavelength bands.
The main goal of the study is to extract information on the clustering, redshift distribution and type of the unresolved non-thermal sources, especially at gamma-ray (and radio) frequencies. We describe how this technique can be used to search for "new" populations of sources, including the one possibly provided by annihilation or decay of particle dark matter.