Dynamical Role of Light Neutral Leptons in Cosmology

Using the Vlasov equation, we show that massive galactic halos cannot be composed of stable neutral leptons of mass \ensuremath{\lesssim} 1 MeV. Since most of the mass in clusters of galaxies probably consists of stripped halos, we conclude that the "missing mass" in clusters does not consist of leptons of mass \ensuremath{\lesssim} 1 MeV (e.g., muon or electron neutrinos). Lee and Weinberg's hypothetical heavy leptons (mass \ensuremath{\lesssim} 1 GeV) are not ruled out by this argument.