  The existence of Dark Matter (DM) is a well established fact since many
decades, thanks to the observation of the effects of its gravitational
interaction with the ordinary matter in the Universe. However, our knowledge of
the Dark Matter features is still rather scarce. Indeed, one of the biggest
quests in fundamental science today is the investigation of Dark Matter nature,
from its origin to its composition, and the way its constituents interact with
the ordinary matter, apart from gravity. Huge and ambitious efforts have been
spent in the last years into its identification, concentrating especially on
the search of viable Weakly Interacting Massive Particle candidates. However,
no positive results have been achieved so far along this direction. On the
other hand, many fascinating new ideas and models for its interpretation have
been blooming: among them, an intriguing hypothesis is that the Dark Matter
constituents could be neutral under Standard Model interactions, but they could
interact through a new, still unknown, force under a "hidden" charge. This new
hidden symmetry would be mediated by a massive gauge boson, the dark photon,
which is expected to couple to the Standard Model via a kinetic mixing. The
search for such a massive mediator has been pursued with large enthusiasm and
dedication in the latest years, as its observation could be within the reach of
many already existing experimental facilities, both based on accelerators
