Are you mixing up anti matter with dark matter? Anti matter is very much real. We make it in labs all the time. It is produced in radioactive decay and by cosmic ray collisions.

No, it's very real and they can even make small amounts of antimatter and store it if they contain it in a magnetic field since contact with matter turns it all into energy. Whenever they hit a couple particles with a super-collider some of it will be antimatter. The only mystery is why the universe turned out to be made of matter which suggests matter might be more stable or created more often somehow.

Edit: Also a theory means something different to scientists and the way people use it in everyday speech would really mean a hypothesis.