DC Universe
The DC Universe is a fictional shared universe in which most stories in American comic book titles published by DC Comics take place. In context, the term "DC Universe" usually refers to the main DC continuity. It contains various superheroes such as Superman, Batman, Wonder Woman, Green Lantern, the Flash, and Aquaman; as well as teams such as the Justice League, the Suicide Squad, and the Teen Titans. It also contains well-known supervillains, including the Joker, Lex Luthor, Brainiac, the Reverse-Flash, and Darkseid.
Beyond the main continuity, the DC Multiverse encompasses all alternate realities within DC Comics. The primary universe has been known by various names over time, with recent designations including "Prime Earth" or "Earth 0". The DC Universe and its alternate realities have been adapted across multiple media, including film serials, radio dramas, and modern films, with ongoing efforts to address the complex continuity through streamlined storylines and events.
History
Golden Age
The fact that DC Comics characters coexisted in the same world was first established in All Star Comics #3 where several superheroes met each other in a group dubbed the Justice Society of America. Earth-Two was the primary world of this publication era, as established in "Flash of Two Worlds" and "Crisis on Earth-Two!".In the Silver Age, the Justice Society was reimagined as the Justice League of America, which was founded with Major League Baseball's National League and American League as inspiration for the name. The comic book that introduced the Justice League was titled The Brave and the Bold. However, the majority of National/DC's publications continued to be written with little regard of maintaining continuity with each other for the first few decades.
Silver Age
In the 1950s and 1960s, DC has introduced different versions of its characters, sometimes presenting them as if the earlier version had never existed, including: Flash, Green Lantern, and Hawkman. These new versions of the characters had similar powers but different names and personal histories. Similarly, they had characters such as Batman whose early adventures set in the 1940s could not easily be reconciled with stories featuring a still-youthful man in the 1960s. To explain this, they introduced the idea of the Multiverse in Flash #123 where the Silver Age Flash met his Golden Age counterpart. In addition to allowing the conflicting stories to "co-exist", it allowed the differing versions of characters to meet, and even team up to combat cross-universe threats. The writers gave designations such as "Earth-One", "Earth-Two", and so forth, to certain universes, designations which at times were also used by the characters themselves. Earth-One was the primary world of this publication era, as established in "Flash of Two Worlds" and "Crisis on Earth-One!".''Crisis on Infinite Earths''
Over the years, as the number of titles published increased and the volume of past stories accumulated, it became increasingly difficult to maintain internal consistency. In the face of diminishing sales, maintaining the status quo of their most popular characters became attractive. Although retcons were used as a way to explain apparent inconsistencies in stories written, editors at DC came to consider the varied continuity of multiple Earths too difficult to keep track of, and feared that it was an obstacle to accessibility for new readers. To address this, they published the cross-universe miniseries Crisis on Infinite Earths in 1985, which merged universes and characters, reducing the Multiverse to a single unnamed universe, a collapsed earth, with a single history.However, not all the books rebooted post-Crisis. For example, the Legion of Superheroes book acted as if the Pre-Crisis Earth-1 history was still their past, a point driven home in the Cosmic Boy miniseries. It also removed the mechanism DC had been using to deal with continuity glitches or storylines that a later writer wanted to ignore resulting in a convoluted explanation for characters like Hawkman.
The Zero Hour limited series gave them an opportunity to revise timelines and rewrite the DC Universe history. However this failed right out of the gate as the writers had Waverider state all alternate histories had been wiped and yet have the Armageddon 2001 saga in the timeline which required multiple timelines to work.
As a result, almost once per decade since the 1980s, the DC Universe experiences a major crisis that allows any number of changes from new versions of characters to appear as a whole reboot of the universe, restarting nominally all the characters into a new and modernized version of their lives.
Meanwhile, DC has published occasional stories called Elseworlds, which often presented alternate versions of its characters. One told the story of Bruce Wayne as a Green Lantern. In another tale, Superman: Speeding Bullets, the rocket ship that brought the infant Superman to Earth was discovered by the Wayne family of Gotham City rather than the Kents.
In 1999, The Kingdom reintroduced a variant of the old Multiverse concept called Hypertime which essentially allows for alternate versions of characters and worlds again. The entire process was possibly inspired by Alan Moore's meta-comic, Supreme: Story of the Year .
The Convergence crossover officially retconned the events of Crisis after heroes in that series went back in time to prevent the collapse of the Multiverse. However, Brainiac states "Each world has evolved but they all still exist". It has been confirmed that all previous worlds and timelines now exist, and that there are even multiple Multiverses now in existence, such as the Pre-Crisis infinite Multiverse, the collapsed Earth, and the Pre-New 52 52 worlds Multiverse.