  We propose that right-handed neutrinos are very long-lived dark matter. The
long lifetime is realized by the separation of the wavefunction of right-handed
neutrinos and that of other fermions in an extra dimension. Such long-lived and
superheavy dark matter can naturally explain observed ultra high energy cosmic
rays above the GZK cutoff (5 * 10^{19} eV) and huge amounts of cold dark matter
simultaneously. Furthermore, the exponentially suppressed Yukawa couplings of
right-handed neutrinos leads to the high predictablilty on the mass parameter
of the neutrinoless double beta decay, as all the models which predict very
small neutrino mass of one generation.
