Antimatter

"My readings indicate you were exposed to an extra-universal material that destroys positive matter on contact, a substance known as antimatter."

- Gideon to Barry Allen

Antimatter is an extra-universal substance that destroys any matter on contact within the multiverse, having the ability to completely destroy a universe and erase it from existence.

Origin
According to Gideon, antimatter is an extra-universal material, meaning that it does not exist within the multiverse, which is mostly made up of matter; rather, antimatter exists outside it, though it is still unknown how the substance is able to make its way into the multiverse.

Non-sentient species, such as pets, can easily pick up on incoming 2. Supergirl noted that all the pets in National City were going nuts before the antimatter even enteres the solar system.

Antimatter walls
In the Speed Force, barriers known as antimatter walls appear when an antimatter wave has a possibility of hitting on a specific date in the respective universe. This can bar a speedster's physical body from time travelling to that date and the few succeeding dates (such as when Barry Allen attempted to time travel to December 11, 2019, a day after a supposed crisis, but was ejected by the Speed Force). Though speedsters in their prime are able to beam their mind to those dates with less lethal consequences, as opposed to inexperienced and/or old-aged speedsters. With the assistance of a machine such as a Neural Hyper-collider, which can harness neural electricity and transfer it across superluminal particles (also known as tachyons) to project one's consciousness across space-time.

Discovery and detection
"For the past year, I've been detecting antimatter signatures across the multiverse. Now, we don't know their origin. But if my algorithm is correct, all Earths could be in danger."

- Jay Garrick

Between 2018 and 2019, Jay Garrick detected a number of antimatter signatures throughout the multiverse. By October 2019, he had marked multiple antimatter signatures on his map, as he detected them near Earths-2, 6, 7, 17, 21, 26, 32, and X.

When Barry Allen attempted to travel to December 11, 2019 in October 2019, an antimatter wall blocked his path, causing the Speed Force to eject Barry back to his native time period, and if not for his accelerated healing factor, the antimatter would've consumed him out of existence. The reasons for the antimatter's presence within the Speed Force were the impending antimatter wave to hit Earth-1 on December 10, 2019 and the uncertainty and numerous possibilities of future events.

Consuming universes
In October 2019, antimatter was unleashed upon Earth-2, killing its population and ultimately destroying the universe. On December 10, 2019, an antimatter wave came to Earths 9, 66, 89, and X. The status of its inhabitants are unknown.

The antimatter wave reached Earth-38 and destroyed Argo City first, leaving Clark Kent, Lois Lane and their son, Jonathan as the only survivors. Although the quantum towers that Mar Novu had established held the wave at bay, the shadow demons of the Anti-Monitor attacked the quantum towers and eventually destroyed it, allowing the antimatter wave to destroy Earth along with the rest of the universe. However, thanks to the coordinated efforts of heroes from both Earth-1 and Earth-38, as well as L-Corp, the D.E.O., the governments of Earth, Harbinger, Novu, and Oliver Queen's sacrifice, 3 billion humans and aliens were able to escape to Earth-1.

Also, on December 10, 2019, the antimatter wave reached an unknown earth, home to Jefferson Pierce/Black Lightning, and as a result, it destroyed the entire universe leaving Jefferson as the only survivor after an unknown force saved him seconds before the wave completely destroyed Freeland.

Possible future
In a potential future (as one of the billions of potential futures viewed by Barry Allen), antimatter made its way into Central City of Earth-1, wiping out the entire universe and its population as a result.

Season 3

 * "The Book of Resistance: Chapter Four: Earth Crisis"

Trivia

 * Antimatter causes planets to have world-wide earthquakes even when outside their solar systems.
 * Antimatter disrupts more complicated forms of magic the stronger its presence is in the multiverse.
 * Constantine noted the ritual to restore Oliver's soul post-Lazarus Pit revival didnt work because of it.
 * Although The Monitor seems to have something of an immunity to antimatter, as he was not killed by an antimatter arrow shot by Oliver, it did bring him to his knees and left him unable to bring Oliver back to Earth-1 until he was fatally injured, suggesting that antimatter weakens the Monitor but doesn't destroy him outright.

Behind the scenes

 * In the real world, antimatter is defined as the "antiparticles" of regular matter found in the universe. Particle accelerators around the world regularly produce tiny amounts, with only a few nanograms having been produced in total.
 * In the DC comics, antimatter is an alternate form of matter comprised of antiparticles. It functions in direct opposition to positive matter and contact between the two will ultimately destroy the particles of both.