They really don't have evidence. What they see in the cosmic microwave backgroud radiation has also been interpreted as possibley being other universes that touched our own. These are clues that may mean nothing. None of these concepts about our universe toughing either old black hoels or other universes are verifiable. They are very very interesting, especially since the 5000 year old Gita has a section that describes the birth, expansion and eventual collapse of our universe, after which it is reborn. I think it was the Gita. It may have been a different mediation based ancient Indian belief system. The idea isn't new.