Physicists and philosophers seem to have fallen in love with the idea of the multiverse: that there are other universes, not spatially connected to our own. There was first the splitting universe idea in quantum mechanics, to deal with the collapse of the wave packet problem, but today with string theory, it seems that everything goes;
https://www.theguardian.com/science/across-the-universe/2017/may/17/multiverse-have-astronomers-found-evidence-of-parallel-universes
https://www.ras.org.uk/news-and-press/2982-new-survey-hints-at-exotic-origin-for-the-cold-spot
http://www.smithsonianmag.com/science-nature/can-physicists-ever-prove-multiverse-real-180958813/
Now, a popular idea of the multiverse is that all realities are actualised. Every possible world is real, in some universe. There is a world where I am not a fat, aging professor of physics in a third-rate US campus, but instead, a young hot-blooded Nobel Prize winner.
To reduce this idea to absurdity, let as consider this possible world: one of the omnipotent multiverse annihilators! If all realities are actualised then there is one possible world where cosmic killers exist, who would seek to destroy all other universes. Argue by assumption that such psychopaths have the power to annihilate universes.
Thus, on the assumption that the multiverse exists, it follows that it would not exist, since the cosmic killers would have annihilated us. Since they have not, they do not exist. It follows that the multiverse does not exist. Physicists need to go back to the drawing board.