  We consider a concise dark matter scenario in the minimal gauged $B-L$
extension of the Standard Model (SM), where the global $B-L$ (baryon number
minus lepton number) symmetry in the SM is gauged, and three generations of
right-handed neutrinos and a $B-L$ Higgs field are introduced. Associated with
the $B-L$ gauge symmetry breaking by a VEV of the $B-L$ Higgs field, the seesaw
mechanism for generating the neutrino mass is automatically implemented after
the electroweak symmetry breaking in the SM. In this model context, we
introduce a $Z_2$-parity and assign an odd parity for one right-handed neutrino
while even parities for the other fields. Therefore, the dark matter candidate
is identified as the right-handed Majorana neutrino with odd $Z_2$ parity,
keeping the minimality of the particle content intact. When the dark matter
particle communicates with the SM particles mainly through the $B-L$ gauge
boson ($Z^\prime_{BL}$ boson), its relic abundance is determined by only three
free parameters, the $B-L$ gauge coupling ($\alpha_{BL}$), the $Z^\prime_{BL}$
boson mass ($m_{Z^\prime}$) and the dark matter mass ($m_{DM}$). With the
cosmological upper bound on the dark matter relic abundance we find a lower
bound on $\alpha_{BL}$ as a function of $m_{Z^\prime}$. On the other hand, we
interpret the recent LHC Run-2 results on search for $Z^\prime$ boson resonance
to an upper bound on $\alpha_{BL}$ as a function of $m_{Z^\prime}$. Combining
the two results we identify an allowed parameter region for this
"$Z^\prime_{BL}$ portal" dark matter scenario, which turns out to be a narrow
window with the lower mass bound of $m_{Z^\prime} > 2.5$ TeV.
