  When the Dark Matter mass is below the eV-scale, its cosmological occupation
number exceeds the ones of photons from the cosmic microwave background as well
as of relic neutrinos. If such Dark Matter decays to pairs of neutrinos, it
implies that experiments that seek the detection of the cosmic neutrino
background may as well be sensitive to this additional form of "dark
radiation". Here we study the prospects for detection taking into account
various options for the forecasted performance of the future PTOLEMY
experiment. From a detailed profile likelihood analysis we find that Dark
Matter decays with lifetime as large as $10^4$ Gyr or a sub-% Dark Matter
fraction decaying today can be discovered. The prospects are facilitated by the
distinct spectral event shape that is introduced from galactic and cosmological
neutrino dark radiation fluxes. In the process we also clarify the importance
of Pauli-blocking in the Dark Matter decay. The scenarios presented in this
work can be considered early physics targets in the development of these
instruments with relaxed demands on performance and energy resolution.
