Antimatter

Antimatter is an extra-universal material that destroys matter on contact within the multiverse, having the ability to completely destroy a universe.

Origins
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 makes its way into the multiverse.

Properties
Presuming from its name, antimatter is the opposite form of matter. When antimatter comes into contact with positive matter, particles of both destroy each other. It may mean that antimatter is relatively harmless when isolated, as opposed to when it touches matter.

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 specific date, though speedsters in their prime are able to beam their mind to that date with less lethal consequences, as opposed to inexperienced and/or old-aged speedsters, with the assistance of a machine such as a neural hypercollider, 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. The reasons for the antimatter's presence within the Speed Force are the impending antimatter wave to hit Earth-1 on December 10, 2019 and the uncertainty and numerous possibilities of future events.

In October 2019, antimatter was unleashed upon Earth-2, killing its population and ultimately destroying the universe.

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

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.