Physicist Hugh Everett's many-worlds interpretation (MWI) of quantum physics implies that there are very many universes, perhaps infinitely many.  MWI views time as a many-branched tree, wherein every possible quantum outcome is realised. This is intended to resolve some paradoxes of quantum theory, such as the two-slit experiment and the Schrödinger’s cat paradox, since every possible outcome of every quantum event exists in its own universe.

In other words, the universe splits every time a quantum event could turn out in two possible ways A or B.  Two new universes are instantly created, one in which A happens and one in which the outcome is B.  Since the quantum world is sub-microscopic and uncounted trillions of events are taking place in every part of the cosmos, there are a lot of new universes being born every second.  A whole lot!

MWI is an extreme violation of the principle of Occam's razor, but it does neatly resolve the apparent paradoxes of modern quantum theory, and as a bonus explains why our universe, against all odds, is perfectly designed so as to allow the existence of matter, stars, planets and our form of life and consciousness.