  DarkSide-20k is a next-generation dual-phase Liquid Argon Time Projection
Chamber, currently under construction at the Gran Sasso National Laboratory
(LNGS) in Italy. With a 20 t fiducial mass of liquid Argon, DarkSide-20k will
probe WIMP-nucleon interactions down to cross sections equal to 10$^{-48}$
cm$^2$ for a WIMP mass of 0.1 TeV/c$^2$. DarkSide-20k is designed to be a
nearly "instrumental background-free" experiment, meaning that less than 0.1
background events are expected in the WIMP search region during the 200
tonne-year planned exposure. To achieve this, the TPC is surrounded by an inner
(neutron) and outer (muon) veto, while low-radioactivity underground argon
(depleted in $^{39}$Ar), is used as the inner detector (TPC and inner veto)
medium. Both the TPC and the veto systems are instrumented with novel cryogenic
silicon photomultiplier, capable of resolving single photoelectrons and
providing the required spatial and time resolution. An overview of the
DarkSide-20k experimental program is reported, with a focus on the
photo-detector system construction and testing procedures for the inner veto
system.
