  Both active and sterile sub-eV neutrinos can serve for hot dark matter (DM).
On the other hand, keV sterile neutrinos could be a good candidate for warm DM.
The beta-decaying (e.g., H-3 and Ru-106) and EC-decaying (e.g., Ho-163) nuclei
are considered as the most promising targets to capture those extremely low
energy neutrinos and antineutrinos, respectively. We calculate the capture
rates of relic electron neutrinos and antineutrinos against the corresponding
beta-decay or EC-decay backgrounds in different flavor mixing schemes. We
stress that such direct laboratory measurements of hot or warm DM might not be
hopeless in the long term.
