Do non-relativistic neutrinos constitute the dark matter?
  The dark matter of the Abell 1689 galaxy cluster is modeled by thermal, non-relativistic gravitating fermions and its galaxies and X-ray gas by isothermal distributions. A fit yields a mass of $h_{70}^{1/2}(12/{\overline g})^{1/4}$1.445 $(30)$ eV. A dark matter fraction $\Omega_\nu=h_{70}^{-3/2}0.1893$ $(39)$ occurs for ${\overline g}=12$ degrees of freedom, i. e., for 3 families of left plus right handed neutrinos with masses $\approx 2^{3/4}G_F^{1/2}m_e^2$. Given a temperature of 0.045 K and a de Broglie length of 0.20 mm, they establish a quantum structure of several million light years across, the largest known in the Universe. The virial $\alpha$-particle temperature of $9.9\pm1.1$ keV$/k_B$ coincides with the average one of X-rays. The results are compatible with neutrino genesis, nucleosynthesis and free streaming. The neutrinos condense on the cluster at redshift $z\sim 28$, thereby causing reionization of the intracluster gas without assistance of heavy stars. The baryons are poor tracers of the dark matter density. 
Th. M. Nieuwenhuizen (Institute for Theoretical Physics, University of   Amsterdam)