Wormholes are purely theoretical. No direct observation of black hole centers has been made, so we have no clue what is inside of them. A wormhole and a different universe are totally nonsensical ideas with no proof or evidence to back them up. For a wormhole to remain stable, it would require immense amounts of negative energy, but negative energy does not exist. Just because wormholes can exist based on the theory of relativity, does not necessarily mean that they do.