  Majorons are the Goldstone bosons of spontaneously broken lepton number and
hence intimately connected to Majorana neutrino masses. Since all majoron
couplings are heavily suppressed by the seesaw scale they are interesting
candidates for long-lived dark matter. The signature decay into two
mono-energetic neutrinos is potentially detectable with neutrino detectors for
majoron masses above MeV and complementary to the loop-induced decays into
visible particles. The mass range between keV and MeV can only be probed
indirectly with the majoron decay into two photons; keV-scale majorons can be
warm or cold dark matter depending on the underlying freeze-in mechanism.
