Many-Worlds' conceptual illustration. The fundamental idea of the Many-Worlds Interpretation (MWI), going back to Everett 1957, is that there are myriads of worlds in the Universe in addition to the world we are aware of.The MWI of quantum mechanics holds that there are many worlds which exist in parallel at the same space and time as our own. The existence of the other worlds makes it possible to remove randomness and action at a distance from quantum theory and thus from all physics. It seems that the majority of the opponents of the MWI reject it because, for them, introducing a very large number of worlds that we do not see is an extreme violation of Ockham's principle: "Entities are not to be multiplied beyond necessity."