  If spacetime torsion couples to the intrinsic spin of matter according to the
Einstein-Cartan-Sciama-Kibble theory of gravity, then the resulting
gravitational repulsion at supranuclear densities prevents the formation of
singularities in black holes. Consequently, the interior of every black hole
becomes a new universe that expands from a nonsingular bounce. We consider
gravitational collapse of fermionic spin-fluid matter with the stiff equation
of state in a stellar black hole. Such a collapse increases the mass of the
matter, which occurs through the Parker-Zel'dovich-Starobinskii quantum
particle production in strong, anisotropic gravitational fields. The subsequent
pair annihilation changes the stiff matter into an ultrarelativistic fluid. We
show that the universe in a black hole of mass $M_\textrm{BH}$ at the bounce
has a mass $M_\textrm{b}\sim M^2_\textrm{BH}
m^{1/2}_\textrm{n}/m^{3/2}_\textrm{Pl}$, where $m_\textrm{n}$ is the mass of a
neutron and $m_\textrm{Pl}$ is the reduced Planck mass. For a typical stellar
black hole, $M_\textrm{b}$ is about $10^{32}$ solar masses, which is $10^6$
larger than the mass of our Universe. As the relativistic black-hole universe
expands, its mass decreases until the universe becomes dominated by
nonrelativistic heavy particles.
