The Universe is defined as the summation of all particles and energy that exist and the space-time in which all events occur. Based on observations of the portion of the universe that is observable, physicists attempt to describe the whole of space-time, including all matter and energy and events which occur, as a single system corresponding to a mathematical model.
The currently accepted scientific theory of the universe’s formation is the Big Bang model, which describes the expansion of space-time from a gravitational singularity. The universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities. Thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in fractal segregation into features that are found in the current universe; such as clusters of galaxies. There are probably more than one hundred billion galaxies in the universe.
A multiverse (or meta-universe) is the hypothetical set of multiple possible universes (including our universe) that together comprise all of physical reality. The different universes within a multiverse are called parallel universes. The structure of the multiverse, the nature of each universe within it and the relationship between the various constituent universes, depend on the specific multiverse hypothesis considered.
Multiverses have been hypothesized in cosmology, physics, philosophy, theology, and fiction, particularly in science fiction and fantasy. In these contexts, parallel universes are also called “alternate universes”, “quantum universes”, “parallel worlds”, or “alternate realities”.