  One of important properties of dark matter is its stability. The U(1)$_{\rm
B-L}$ gauge symmetry is the most attractive symmetry to guarantee the
stability. Though the symmetry is expected to be broken at very high energy
scale to account for tiny neutrino masses through the seesaw mechanism, the
residual discrete symmetry of U(1)$_{\rm B-L}$ can stabilize the dark matter
naturally. We prove that, when there is new physics connecting B$-$L charges of
dark matter and standard model particles at the scale between the electroweak
and the U(1)$_{\rm B-L}$ breaking, the mass of dark matter is definitely
predicted to be (5--7)/$Q_{\rm DM}$ GeV ($Q_{DM}$ is the B$-$L charge of dark
matter) independent not only of details of the new physics but also of its
energy scale. We also show two attractive examples. First one is the scalar
dark matter with the B$-$L charge of one, which turns out to be very consistent
with current CoGeNT results. Another one is the fermionic dark matter having
the B$-$L charge of one third, which is also attractive from the viewpoint of
model building.
