Pretty much any time subatomic particles interact with each other or with anything else anywhere in the universe, there are multiple possible outcomes, which means a lot of branching of history. There may even be infinitely many branches. So is the many-worlds hypothesis true? We don't know.

If the final theory of everything is non-linear with respect to wavefunctions, then many-worlds is invalid. All quantum field theories are linear and compatible with the MWI, a point Everett emphasized as a motivation for the MWI.

One of the things which seems to make the standard theory unsuitable for a realist reading is the fact that there are two evolution equations, and that they are expressed in terms of measurement: the Schrödinger evolution holds when no measurements are performed, and the collapse rule when a measurement takes place.

At present, there is no candidate theory of everything that includes the standard model of particle physics and general relativity and that, at the same time, is able to calculate the fine-structure constant or the mass of the electron.