  In this review article, we consider a dark matter scenario in the context of
the minimal extension of the Standard Model (SM) with a $B-L$ (baryon number
minus lepton number) gauge symmetry, where three right-handed neutrinos with a
$B-L$ charge $-1$ and a $B-L$ Higgs field with a $B-L$ charge $+2$ are
introduced to make the model anomaly-free and to break the $B-L$ gauge
symmetry, respectively. The $B-L$ gauge symmetry breaking generates the
Majorana masses for the right-handed neutrinos. We introduce a Z$_2$ symmetry
to the model and assign an odd parity only for one right-handed neutrino, and
hence the Z$_2$-odd right-handed neutrino is stable and the unique dark matter
candidate in the model. The so-called minimal seesaw works with the other two
right-handed neutrinos and reproduces the current neutrino oscillation data. We
consider the case that the dark matter particle communicates with the SM
particles through the $B-L$ gauge boson ($Z^{\prime}_{B-L}$ boson), and obtain
a lower bound on the $B-L$ gauge coupling ($\alpha_{B-L}$) as a function of the
$Z^{\prime}_{B-L}$ boson mass ($m_{Z^{\prime}}$) from the observed dark matter
relic density. On the other hand, we interpret the recent LHC Run-2 results on
the search for a $Z^{\prime}$ boson resonance to an upper bound on
$\alpha_{B-L}$ as a function of $m_{Z^{\prime}}$. These two constraints are
complementary to narrow down an allowed parameter region for this "$Z^{\prime}$
portal" dark matter scenario, leading to a lower mass bound of $m_{Z^{\prime}}
\geq 3.9$ TeV.
