Every black hole may hold a hidden universe : "For his analysis, Poplawski turned to a variant of GR called the Einstein-Cartan-Kibble-Sciama (ECKS) theory of gravity. Unlike Einstein's equations, ECKS gravity takes account of the spin or angular momentum of elementary particles. Including the spin of matter makes it possible to calculate a property of the geometry of space-time called torsion.
When the density of matter reaches gargantuan proportions (more than about 1050 kilograms per cubic metre) inside a black hole, torsion manifests itself as a force that counters gravity. This prevents matter compressing indefinitely to reach infinite density, so there is no singularity. Instead, says Poplawski, matter rebounds and starts expanding again."