Before the Big Bang, around 13.8 billion years ago, some physicists have theorised that all the mass and energy of the universe in which we reside was compacted into an incredibly dense and tiny “seed” formed inside a black hole under the intense heat, pressure, and gravity of a giant collapsed star.
Einstein proposed that this centre of a black hole would be infinitesimally small and have gravity so strong that even light cannot escape, but if it was unobservably small, but finite, then that raises the possibility that it could be the seed from which everything we can now observe sprung forth.
An article in the latest issue of National Geographic expands on this idea, as proposed by Nikodem Poplawski, in discussing the possibility that a “seed” smaller than any atom observed by humans could contain all the energy and mass needed to create an entire galaxy.
Such discussions are highly speculative and currently unprovable, but the idea that our universe resides within a single black hole would support the popular multiverse theory, in where our universe is just one of a huge number of seperate universes possibly linked to each other in some way.