  We review the recent proposal by Lattanzi & Valle of the majoron as a
suitable warm dark matter candidate. The majoron is the Goldstone boson
associated to the spontaneous breaking of ungauged lepton number, one of the
mechanisms proposed to give rise to neutrino masses. The majoron can acquire a
mass through quantum gravity effects, and can possibly account for the observed
dark matter component of the Universe. We present constraints on the majoron
lifetime, mass and abundance obtained by the analysis of the cosmic microwave
background data. We find that, in the case of thermal production, the limits
for the majoron mass read 0.12 keV<m_J<0.17 keV, and discuss how these limits
are modified in the non-thermal case. The majoron lifetime is constrained to be
larger than 250 Gyrs. We also apply this results to a given seesaw model for
the generation of neutrino masses, and find that this constraints the energy
scale for the lepton number breaking phase transition to be above 10^6 GeV. We
thus find that the majoron decaying dark matter (DDM) scenario fits nicely in
models where neutrino masses arise "a la seesaw" and may lead to other possible
cosmological implications.
