Basically, the equations of quantum Physics are fixed, but there are many ways to "interpret" or give meaning to them. The classical one, called Copenhagen, asserts a "wavefunction collapse" that would happen on ill-defined conditions, and is problematic as we find experimentally larger and larger quantum systems.
Another interpretation, Many Worlds (or its more mathematically rigorous form, Decoherence) does not introduce any new concept other than the basic quantum physics equations and has no problem with large systems. In fact, under the Decoherence / Many World interpretation, the entire universe is a single quantum system that has been evolving according to the wave functions since the beginning of time.
I've written more about it above.