  Dark matter stability can result from a residual matter-parity symmetry,
following naturally from the spontaneous breaking of the gauge symmetry. Here
we explore this idea in the context of the $\mathrm{SU(3)_c \otimes SU(3)_L
\otimes U(1)_X \otimes U(1)_{N}}$ electroweak extension of the standard model.
The key feature of our new scotogenic dark matter theory is the use of a
triplet scalar boson with anti-symmetric Yukawa couplings. This naturally
implies that one of the light neutrinos is massless and, as a result, there is
a lower bound for the $\rm 0\nu\beta\beta$ decay rate.
