Cosmological data is reviewed suggesting that the universe may be open and dominated by neutrinos and gravitons rather than by baryons. The thermal history of the Lepton Era is investigated incorporating the effects of neutral currents, additional neutrinos, and a small neutrino mass. In the canonical version of Big Bang cosmology (equal numbers of neutrinos and antineutrinos), the neutrino number and energy density is, like that of photons, gravitationally insignificant unless the neutrino has a small mass (∼10 ev).The neutrino sea can be cosmologically significant if it is degenerate (so that the net leptonic or muonic charge is non‐zero) with∼ 7 × 105neutrinos (or anti‐neutrinos) per cc. This density homogeneously spread out is still so low that even the most energetic cosmic ray protons will not be stopped, even if neutral currents exist with the usual weak strength. If these degenerate neutrinos have a small mass ∼0.5 ev, they will condense into degenerate neutrino superstars of the size and mass of galactic clusters.If neutral currents make the (e&ngr;)(e&ngr;) coupling 5 × greater than what it is in V‐A theory, nucleosynthesis commences a little earlier than conventionally assumed. This increases the cosmological He4abundance predicted only slightly fromY = 0.27toY = 0.29.An appendix reviews the effect of neutral currents on neutrino processes in stars.