  An attractive way to generate neutrino masses as required to account for
current neutrino oscillation data involves the spontaneous breaking of lepton
number. The resulting majoron may pick up a mass due to gravity. If its mass
lies in the kilovolt scale, the majoron can play the role of late-decaying Dark
Matter (LDDM), decaying mainly to neutrinos. In general the majoron has also a
sub-dominant decay to two photons leading to a mono-energetic emission line
which can be used as a test of the LDDM scenario. We compare expected photon
emission rates with observations in order to obtain model independent
restrictions on the relevant parameters. We also illustrate the resulting
sensitivities within an explicit seesaw realisation, where the majoron couples
to photons due to the presence of a Higgs triplet.
