Stephen Hawking's final work is the basis for the experimental demonstration of the multiverse theory. What is the theory? The universe is expanding. It is easy to observe that stars are moving further away in every direction. We can compare it to the Italian Christmas panettone where, during the leavening process, all the single pieces of candied fruits move away from each other as the whole panettone rises. At the beginning the universe was smaller, denser and warmer. It was a singularity, meaning a density so extreme that it is indescribable. At some point this singularity exploded and this is the big-bang.
But what caused the big bang? The multiverse theory attempts to answer the question. According to the theory, before the big-bang the universe was made of energy of a different form and was expanding anyway but without decreasing in density. Imagine our panettone multiplying instead of rising, staying at the same density as if we were adding new dough to it. The big bang occurred when a part stopped multiplying with the others, generating a singularity with the physical law features we know of today. This starts to expand like our rising panettone, creating an area of non-inflationary time space, forever separated from the inflationary universe and also from other non-inflationary universes. Yes, other universes like ours, but in other regions of space time, are generated every moment by the inflationary universe.