What is antimatter?
Antimatter is matter with the same mass and every charge reversed. A particle here is a knot in the way neighboring patches of the world translate each other's readings, a twist that no local repair can undo. Every twist the wiring allows comes with a mirror twist, and the mirror is the antiparticle. Bring the two together and the twists cancel, the knot comes apart and its upkeep is released, which is annihilation. Paul Dirac predicted the positron, the anti-electron, in 1931, and Carl Anderson photographed one in 1932. Hospital scanners photograph the pair of flashes from each positron annihilating. The mirror is slightly imperfect. In 1964 about one in five hundred long-lived neutral kaons decayed the way an exact matter-antimatter symmetry forbids, and a universe with any matter left in it needs that difference.
Also in Chapter 28: What Is a Particle, What Is a Wave, and Why Does It Look Like Both?.
This answer comes from OPH, Observer Patch Holography. OPH is a rigorous, consistent and complete description of the unified, self-creating computational Universe we inhabit. Its consistency is machine-checked: the fixed point at its core is constructed explicitly, and a theory with a model contains no contradiction.
To understand OPH and check every step yourself, start with the free book Reverse Engineering Reality. It builds the theory from the ground up, and the OPH Notebook answers your questions while you read.
read the free bookask the notebookor ask sage:read chapter 27: Why This Exact List of Matter, and Why Three Times Over?watch a universe make its own particles