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.
If the former is true, then the unknown substance may have formed a barrier in our Universe, creating a "Baryonic-Universe" on one side and "Anti-Universe" on the other. Antimatter is usually very small, with antimatter particles being up to 20 orders of magnitude smaller than a normal particle, . 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.