  We exhibit a new instability that leads to "fast" coherent
neutrino-antineutrino mixing in a dense neutrino cloud, arising from the
standard four-Fermi interaction induced by Z exchange, and overlooked in the
big existing literature on fast processes in this venue. It can play an
essential role in creation of an abundance of pseudoscalar mesons of mass in
the KeV range in the early universe, at temperatures in the range 10-100 MeV.
Also it can overpower existing calculations in the "neutrino bulb" regions of
the supernova cloud.
