One theory about the multiverse is called the ‘Many-Worlds’ interpretation of quantum physics. It proposes that every time a quantum measurement is made, the universe splits into multiple parallel universes, each representing a different outcome of that measurement.

Another theory is the ‘Inflationary’ theory, which suggests that in the early moments of our universe, it underwent a period of rapid expansion, causing regions of space-time to peel off and become universes of their own.

Lastly, the ‘String Theory’ proposes there may be many universes in higher dimensions, with different types of matter and forces than what our universe is made of.