  Under rather generic assumptions, we show that in the asymmetric dark matter
(ADM) scenario, the sign of the B-L asymmetry stored in the dark matter sector
and the standard model sector are always the same. One particularly striking
consequence of this result is that, when the dark matter decays or annihilates
in the present universe, the resulting final state always involves an
anti-neutrino. As a concrete example of this, we construct a composite ADM
model and explore the feasibility of detecting such an anti-neutrino signal in
atmospheric neutrino detectors.
