Multiverse

The multiverse is a concept that refers to the existence of infinite universes that comprise everything that is. Each universe within the multiverse is referred to as a different "Earth", and each vibrates at a different frequency so that they cannot normally interact with or be seen by each other. If one is able to travel fast enough it is possible to breach the laws of physics and travel between these Earths.

History
"This is our earth. Let's call it Earth-1. And this second Earth, let's call it Earth-2. This is where Jay claims to be from. And these other earths, three, four, five, to infinity, all of them, are nearly identical to ours. And they all exist at the same time, giving us endless alternatives to what we have here. For example, the Joe West on Earth-2, your doppelganger, he might not be a detective. He could be a Nobel Prize-winning physicist. And you, the Barry Allen here is The Flash, but over there, he could be an electrician."

- Martin Stein explaining the multiverse

In early 2015, Barry Allen and the rest of Team Flash accidentally opened a singularity above Central City; though they succeeded in closing it, a series of 52 breaches between Earths One and Two were opened throughout the city as a result of their actions.

Between mid to late 2015 and early 2016, an Earth Two speedster known as Zoom began exploring the Multiverse before using the breaches to launch an assault on Earth One's Central City in an attempt to steal The Flash's speed. Zoom sent numerous meta-humans through the breaches, each tasked with killing The Flash, but none succeeded. Team Flash, in an effort to stop Zoom's onslaught, systematically sealed each of the breaches, leaving only one open on a lower level inside S.T.A.R. Labs. Barry, Cisco Ramon, and Harrison Wells used the breach to travel to Earth Two in order to rescue Jesse Quick, sealing the breach upon their return.

Attempting to increase his speed, Barry Allen used a tachyon device, which in turn caused him to run too fast and accidentally create a breach between Earth One and Earth Thirty-Eight, the parallel world that is the home of Supergirl.

During his travels, Zolomon discovered that Earth One happened to be in the center of the entire Multiverse, and that from there one could travel to any of the other infinite number of Earths. Enthralled by the idea of conquering all of existence, in May of 2016 he attempted to use the Mercury Labs Magnetar to instantaneously destroy every Earth in the Multiverse besides Earth One, using its central location as a means of targeting every other universe at the same time. Through the sacrifice of his own time remnant, Barry Allen was able to stop the device moments before it destroyed the Multiverse, and also managed to defeat Zolomon in the process.

Known parallel worlds

 * Earth One: The central Earth of the multiverse, home to Team Flash, Team Arrow, John Constantine's team and the Legends.
 * Earth Two: A darker Earth home to the deadly speedster Zoom.
 * Earth Three: An anachronistic Earth home to Jay Garrick, a more seasoned incarnation of The Flash.
 * Earth Sixteen: An Earth Cisco Ramon briefly made contact with.
 * Earth Seventeen: A steampunk-flavored alternate Earth.
 * Earth Nineteen: The native Earth of "HR" Wells.
 * Earth Thirty-Seven: An Earth Cisco Ramon briefly made contact with.
 * Earth Thirty-Eight: An Earth where alien life has long been public knowledge, home to the heroes Supergirl and Superman.
 * An unnamed Earth (home to CBS' The Flash)
 * An unidentified Earth (home to Hells Wells)
 * An unidentified Earth (home to the French-speaking Harrison Wells)

Known constants in the multiverse

 * Harrison Wells
 * S.T.A.R Labs
 * Big Belly Burger

Trivia

 * The fact that the number of breaches that opened in Central City is 52 can be seen as a possible reference to both the multiverse crisis storyline known as 52 and the New 52 imprint in the DC Comics.