AnnihilationAnnihilation occurs when a particle collides with an antiparticle. It transforms mass into energy of some form. The energy is carried by force carriers, particles that can decay into other particles. It means that, contrary to a popular belief, the annihilation produces more than photons. What is produced depends on the particles and antiparticles involved.
When an electron annihilates a positron (anti-electron) the process yields to pure energy in a form of gamma rays. When a proton annihilates an antiproton they produce gamma rays and a swarm of secondary particles, like pairs of top-anti-top quarks. The secondary particles will eventually decay into neutrinos and low-energy gamma rays. Knowing that neutrinos could hold some mass, it could mean that the annihilation doesn't transform all the mass into energy.