Antimatter

Antimatter is a substance found in our Universe that has both opposite charge and spin to baryonic matter. When meeting baryonic matter, it will explode along with the baryonic matter in a so-called annihilation. If the universe was made up of half-antimatter and half-matter, then they would have all detonated leaving nothing except for radiation behind, so the theory is that either there is a third substance that stops antimatter-matter annihilation, or somehow more matter exists than antimatter.

Properties
Antimatter is usually very small, with antimatter particles being up to 20 orders of magnitude smaller than a normal particle. It is also much less massive. Otherwise, it is identical to baryonic matter if left in a vacuum, and has no special properties, so the existence of antimatter entities is possible.

Alternatively
Anti-Matter is the material used to make any and all types of antiverses out there. It is said that those made of Anti-Matter are not aware of how unorthodox their world seems and that they in their minds acknowledge everything as flowing ever so correctly. On the outside there can be "glitches" and sorts