- 'F. Massaro, R. D’Abrusco, G. Tosti, M. Ajello, A. Paggi, D. Gasparrini.'
title: 'Unidentified gamma-ray sources: hunting $\gamma$-ray blazars'
---

Introduction {#sec:intro}
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More than half of the $\gamma$-ray sources detected by [*Compton*]{} Gamma-Ray Observatory (CGRO), and present in the third EGRET (3EG) catalog were not associated with known counterparts seen at low energies [@hartman99]. Whatever the nature of the unidentified $\gamma$-ray sources (UGSs), these objects could provide a significant contribution to the isotropic gamma-ray background (IGRB) [e.g., @abdo10a]. Solving the puzzle of the origin of the UGSs, together with a better knowledge of other IGRB contributions estimated from known sources, is crucial also to constrain exotic high-energy physics phenomena, such as dark matter signatures, or new classes of sources.

With the advent of the [[*Fermi*]{}]{} mission the localization of $\gamma$-ray sources has significantly improved with respect to the past $\gamma$-ray missions, thus simplifying the task of finding statistically probably counterparts at lower energies. New association methods also have been developed and applied, so that the number of UGSs has significantly decreased with respect to the 3EG catalog [@hartman99]; however, according to the second [[*Fermi*]{}]{} $\gamma$-ray catalog (2FGL), about one third of detected gamma-ray sources in the energy range 