On a Time Variation of Neutrino's Mass

After the introduction to the subject, we review the Machian relations involving the different kinds of energy densities in the Universe. By suposing that dark-matter is composed by Neutrino's massive energy, we hint that the Neutrino's masses should be proportional to the inverse of R, while the total number of Neutrinos in the Universe increases with the square of R, where R stands for Hubble's length. This makes possible that the dark mater-energy density be of Neutrinos origin, to vary with the inverse square of R, as is necessary for a Machian Universe. The neutrino average mass is estimated for the present Universe (=10 to the minus 76 power, in grams).