I assume that dark matter is made of heavy neutrinos.
One of the models for constituents of dark matter uses supersymmetry with R parity conservation. The corresponding supersummetric partners of neutrinos are called neutralinos n. There are problems with these models and it is a research field.
But wouldnt they be converted into neutrinos with less mass throygh weak interaction
No ,neutralinos cannot decay because of R parity conservation.
Neutralinos are beyond the standard model that fits observed particles, so it is a new particle, if it exists.
Everything about dark matter is under research.