Many Worlds exist in parallel in the same space & time as our own.

The Many Worlds Interpretation (MWI) is a philosophical and mathematical position about how the mathematics used in quantum mechanics relates to physical reality. It asserts that the universal wave function is objectively real, and that there is no wave function collapse. This implies that all possible outcomes of quantum measurements are physically realized in some "Many Worlds" or universe.

In contrast to some other interpretations of quantum mechanics, the evolution of reality as a whole in Many Worlds is rigidly deterministic. Many worlds is also called the relative state formulation or the Everett interpretation, after physicist Hugh Everett, who first proposed it in 1957. Bryce DeWitt popularized the formulation and named it many worlds in the 1970s.

In modern versions of many worlds, the subjective appearance of wave function collapse is explained by the mechanism of quantum decoherence. Decoherence approaches to interpreting quantum theory have been widely explored and developed since the 1970s.

The many worlds interpretation implies that there are most likely an uncountable number of universes. It is one of a number of multiverse hypotheses in physics and philosophy. Many Worlds views time as a many-branched tree, wherein every possible quantum outcome is realized. This is intended to resolve some paradoxes of quantum theory, such as Wigner's friend, the EPR paradox, and Schrödinger's cat, since every possible outcome of a quantum event exists in its own universe.

The Many Worlds Interpretation proposes that all of quantum mechanics is just wave functions smoothly evolving through time. There are no extra rules for collapsing wave functions, and no notion of measurement is fundamentally involved in this interpretation.

It's worth noting that the Many Worlds Interpretation is not the only interpretation of quantum mechanics. There are other interpretations, such as the Copenhagen interpretation and hidden variable theories like Bohmian mechanics. Each interpretation has its own strengths and weaknesses, and the debate among physicists about which one is correct continues to this day.

SOURCE
The Why Files

Many-worlds interpretation - Wikipedia
https://en.wikipedia.org/wiki/Many-worlds_interpretation