A group of multiple universes that exist close to one another but are vastly different. This is where most of my stories are set.

Sci-fi/fantasy

No one knows how the multiverse came to be. Nobody is sure which universe came first, and how it happened. It just wasn't, and then it was. From then on, there were Celestial beings (often called Gods) who were assigned to each universe. It took quite some time, but eventually, each Celestial crafted their own universe by manipulating energy.

Depends on the universe.

Each universe must have laws and limitations, or it will fall apart. In the human universe, these are the laws of physics. In the senseless universe, there are some basic laws of physics as well as laws of hierarchy (a creature’s status determines how they fit in and interact with the universe and their connection with it. Creatures of higher status have more freedom. Creatures of low status have little freedom to interact with their universe and have little connection with it. A creature’s status is also determined by their species, and cannot be changed.)

Any creature has the ability to jump from universe to universe, either by using a dimensional mirror or something else I haven’t come up with yet. However, there is no guarantee that that creature will survive. It is actually very likely that the creature will be immediately rejected by that universe and killed instantly. There are some creatures that typically survive in the majority of universes, such as humans/humanoids, fish, and insects. Creatures that are unique to their universe typically don’t survive anywhere else. Celestial beings, however, survive in every and any universe. Instead of being damaged by universes that don’t recognize their form/chemistry/energy, their presence can damage the universe that they are in. Beyond that, it is up to the Celestial who controls the universe to craft a magic system. After the magic system is created, the universe is automatically split into two identical parts: one with magic and one without it. No one knows why this happens.

The multiverse is made of one type of energy that can be converted into any type of energy- celestial energy. In the human universe, this is what makes atoms, molecules, and elements.
Each universe has an assigned celestial being. Sometimes it has several, such as in the senseless universe. Each celestial being has an equal amount of power and the ability to manipulate their universe however they wish.
Celestial beings can be banned from any universe by the being that created it.