  Temporary matter domination and late entropy dilution, injected by a
"long-lived" particle in the early universe, serves as a standard mechanism for
yielding the correct dark matter relic density. We recently pointed out the
cosmological significance of diluting particle's partial decay into dark
matter. When repopulated in such a way, dark matter carries higher momentum
than its thermal counterpart, resulting in a suppression of the linear matter
power spectrum that is constrained by the large scale structure observations.
In this work, we study the impact of such constraints on the minimal left-right
symmetric model that accounts for the origin of neutrino mass. We map out a
systematic anatomy of possible dilution scenarios with viable parameter spaces,
allowed by cosmology and various astrophysical and terrestrial constraints. We
show that to accommodate the observed dark matter relic abundance the
spontaneous left-right symmetry breaking scale must be above PeV and cosmology
will continue to provide the most sensitive probes of it. In case the dilutor
is one of the heavier right-handed neutrinos, it can be much lighter and lie
near the electroweak scale.
