Antimatter in matter composed of the analogous antiparticles to ordinary matter. Physicists believe symmetry
breakdown at the most fundamental levels after the Big
Bang resulted in our universe where matter predominates and anti-matter is all but non-existant.
Anti-hydrogen consists of an anti-electron
aka Positron, orbiting an Antiproton. Truly miniscule amounts of Anti-hydrogen (a few atoms) have been made in the largest accelerator laboratories.
When reacted with ordinary matter anti-matter would
indeed release enormous amounts of energy according to E=MC^2. However, much of this energy is carried away in neutrinos, which interact so slightly with anything at all, matter or antimatter, capturing the output of a M-AM reactor would present another huge hurdle.
However, it
may not necessarily take more energy input than output. This is based on the misconception that the input to the accelerators is the same energy as released by the reaction. The energy released by a
future M-AM reactor comes from annihilation, rather than being stored energy from the accelerator
drive & containment. Most of the huge power requirements of accelerators goes into their magnetic confinement
system. Suppose it becomes possible to
re-arrange the quarks constituting
normal matter into the configuration of antimatter much more efficiently. This is a big science
fiction IF, by the way. Thus if a “matter inverter” were to function reasonably efficiently, the downstream M-AM reactor would see a net gain.
One possibility would be to base the “matter inverter” on a hypothetical Bussard Ramjet. This would be a magnetic or electrostatic “scoop” that collects interstellar hydrogen to power a hypothetical interstellar spacecraft at relativistic speeds. An accelerator might not be needed, as the hydrogen & whatever other atoms are out there would be
running into the craft at near-light
speed already. Although the Bussard scoop field(s) would probably
use a lot of power in its own right.
Remember and be warned, when one extrapolates science far enough beyond the frontiers of present knowledge, it becomes largely indistinguishable from science
fiction!