  The most massive black holes, lurking at the centers of large galaxies, must
have formed less than a billion years after the big bang, as they are visible
today in the form of bright quasars at redshift larger than six. Their early
appearance is mysterious, because the radiation pressure, generated by
infalling ionized matter, inhibits the rapid growth of these black holes from
stellar-mass black holes. Here we show that the supermassive black holes may
form timeously through the accretion of radiationless and densely packed
neutrino dark matter onto stellar-mass black holes. Our symbiotic scenario
relies on the formation of, first, supermassive degenerate sterile neutrino
balls through gravitational cooling and, then, stellar-mass black holes through
supernova explosions of massive stars at the center of the neutrino balls. The
observed lower and upper limits of the supermassive black holes are explained
by the corresponding mass limits of the preformed neutrino balls.
