If dark matter ( DM ) is unstable , in order to be present today , its lifetime needs to be longer than the age of the Universe , t _ { U } \simeq 4 \times 10 ^ { 17 } s. It is usually assumed that if DM decays it would do it with some strength through a radiative mode .
In this case , very constraining limits can be obtained from observations of the diffuse gamma ray background .
However , although reasonable , this is a model-dependent assumption .
Here our only assumption is that DM decays into , at least , one Standard Model ( SM ) particle .
Among these , neutrinos are the least detectable ones .
Hence , if we assume that the only SM decay daughters are neutrinos , a limit on their flux from DM decays in the Milky Way sets a conservative , but stringent and model-independent bound on its lifetime .