My understanding (which is admittedly limited) was that the main thing holding back the theory of multiverses was the uncertainty of inflation, if it was "real" or not. Since inflation has been confirmed now we can postulate that the "big bang" was not an isolated occurance and may be just a small part of the overall picture.
As space "grows" energy fills in the "emptiness", which in turn may lead to the creation of "new" universes via new "big bangs"... If I understand it correctly? Of course this is simplified
Is the main guy studying this and there are also two Russian physicists working on the theory as well but they're names escape me at this time