The observed dark matter distribution of the supercluster Abell 1689 is modelled by a thermal distribution of dark matter particles. The fit points to fermions, with mass $(g/12)^{1/4}$ 1.57 eV. A dark matter fraction 0.204 can be achieved for statistical weight g=12, corresponding to left plus right-handed neutrinos and anti-neutrinos in three families (Dirac fermions). In the center of the supercluster they set up a quantum structure of, say, a million light years. To achieve thermal equilibrium, a scattering cross section of order $10^{-37}$ m$^2$ is needed, much larger the value in the standard model. If the standard model value pertains, our approach is still an estimate for free streaming neutrinos, that condense gravitationally during expansion. In nucleosynthesis the right-handed neutrinos cause too much $^4$He, but this can be compensated by assuming a neutrino asymmetry.

Institute for Theoretical Physics, University of Amsterdam