Well Its very compliceted to reply a query which requires a substantial knowledge of quantum mechanics and General theory of relativity . Understanding with out mathemtics is a story so the story is The concept of paralell Universe came up due to the fundamental rules governing our universe the rules which were originated from quantum mechanics Like photon stays in two states at the same time two states observed and obeys their own set of rule then we can say there might be two universe at the same time or many .
Even with relativity theory, gaps in or knowledge concerning matter and spacetime still exist. Our present models of the beginning of time called cosmological theories still carry a Newtonian mechanical tinge. They still ring of clockworks and questions about what happened before the big bang--the so-called beginning of everything--paradoxically ring in our heads. And the present models still are grappling with how to bring quantum physics into the beginning of space, time, and matter.
This all seems very far-fetched from Einstein's theory, but Einstein actually speculated and believed in parallel universes. He helped to discover the Einstein-Rosem Bridge-the area at the singularity of a rotating black hole where one can cross into a parallel universe.
Hope this helps u and our able researchers prove the existence of parallel universe