Parallel universes in fiction, a hypothetical self-contained plane of existence, co-existing with one's own. Alternate history, a genre of fiction in which historical events differ from reality. Alternative universe (fan fiction), fiction by fan authors that departs from the fictional universe of the source work.
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Does a parallel universe exist?
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What is the difference between parallel and alternate universe?
Parallel universes in fiction
A parallel universe, also known as an alternate universe, parallel world, parallel dimension, alternate reality, or alternative dimension, is a hypothetical self-contained layer or plane of existence, co-existing with one's own. Wikipedia