  Unstable relics with lifetime longer than the age of the Universe could be
the dark matter today. Electrons, photons and neutrinos are a natural outcome
of their decay and could be searched for in cosmic rays and in $\gamma$-ray and
neutrino detectors. I compare the sensitivities of these three types of
searches to the mass and lifetime of a generic unstable particle. I show that
if the relics constitute our galactic halo and their branching ratios into
electron-positrons, photons and neutrinos are comparable, neutrino searches
would probe the longest lifetimes for masses $\simge 40 \TeV$, while
electron-positron searches would be better but more uncertain for lighter
particles. If instead the relics are not clustered in our halo, neutrinos are
more sensitive a probe than $\gamma$-rays for masses $\simge 700 \GeV$. A $ 1
\sqkm $ neutrino telescope should be able to explore lifetimes up to $ \sim
10^{30} \sec $ while searching for neutrinos from unstable particles above the
atmospheric background.

High Energy Physics - Phenomenology