Page "Marvel Universe" Paragraph 69
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Wikipedia
One cannot normally alter the Marvel Universe's history ; if a time-traveller should cause an alteration to the established flow of events at some point in the past, a divergent universe will simply " branch out " from the existing timeline, and the time-traveller will still return to his or her unaltered original universe.
It is unknown why this happens, though a warp known as the Nexus of All Realities exists in a swamp in the Florida of Earth-616.
For the most part this does not matter, as most beings are unaware that this occurs, or even that their universes were recently " born " from another.
However, individuals and organizations exist that try to monitor or manipulate the various realities.
These include Immortus, the Captain Britain Corps, the Time Variance Authority, the Timebreakers / Exiles, and Kang the Conqueror's forces.
It has been shown to be possible to travel through time without creating a new alternative universe, instead altering events in the future, but this seems to have devastating and very far-reaching repercussions, as depicted in Marvel 1602 ( it almost destroyed the whole multiverse, including the afterlife ).
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