science

At this tiny scale, energy and mass are essentially equivalent. So when we say that matter annihilates, we mean that they transform into pure energy (in this case, as photons of light). They don't break into subatomic particles, because that still counts as mass. They just simply cease to exist.

As a side note, the "conversion rate" of mass into energy (and vice versa) is governed by Einstein's E=mc^2. All this equation means is that it takes a ridiculous amount of energy to create a small amount of mass, and vice versa, it only takes a small amount of mass to create a ridiculous amount of energy. Because antimatter annihilates completely (ie, 100% of its mass, as well as 100% of the regular matter's mass, gets converted into energy), antimatter is currently the most explosive thing known to mankind