  While the formation of the first black holes at high redshift is reasonably
well understood though debated, massive black hole formation at later cosmic
epochs has not been adequately explored. We present a gas accretion driven
mechanism that can build up black hole masses rapidly in dense, gas-rich
nuclear star clusters (NSCs). Wind-fed supra-exponential accretion of an
initially wandering black hole in NSCs can lead to extremely fast growth,
scaling stellar mass remnant seed black holes up to intermediate mass black
holes (IMBHs). Operating throughout cosmic time, growth via this new channel is
modulated by the gas supply, and premature termination results in the formation
of lower mass black holes with masses in the range of 50 - few 100 solar
masses, filling in the so-called mass gap. However, in most gas-rich NSCs,
growth is unimpeded, inevitably leading to the formation of IMBHs with masses
ranging from 100 - 100,000 solar masses. A spate of new detection spanning the
full range of the IMBH mass function - from the LIGO-VIRGO source GW190521 to
the emerging population of 10^5 solar mass black holes harbored in low-mass
dwarf galaxies - are revealing this elusive population. Naturally accounting
for the detected presence of off-center IMBHs in low-mass dwarfs, this new
pathway also predicts the existence of an extensive population of wandering
non-central black holes in more massive galaxies would be detectable via tidal
disruption events and as GW sources. Gas-rich NSCs serve as incubators for the
continual formation of black holes over a wide range in mass throughout cosmic
time.
