  Late time decay of very heavy dark matter is considered as one of the
possible explanations for diffuse PeV neutrinos observed in IceCube. We
consider implications of multimessenger constraints, and show that proposed
models are marginally consistent with the diffuse gamma-ray background data.
Critical tests are possible by a detailed analysis and identification of the
sub-TeV isotropic diffuse gamma-ray data observed by Fermi and future
observations of sub-PeV gamma rays by observatories like HAWC or Tibet AS+MD.
In addition, with several-year observations by next-generation telescopes such
as IceCube-Gen2, muon neutrino searches for nearby dark matter halos such as
the Virgo cluster should allow us to rule out or support the dark matter
models, independently of gamma-ray and anisotropy tests.
