Fictional universes are most often used in science fiction and fantasy stories but they can be used in any type of fiction. A fictional universe usually has certain things in them that make them different from the real world. These things could affect anything from cities the author created to entire galaxies. In most fictional universes, the laws of physics are different to let things exist which commonly do not, for example, magic or space travel to other planets. Fictional universes may also take place in any time period during the past, present, or future.