Annihilation of positron and eletron particles

Annihilation of positron and electron is a clear example of the deep
distance between empiric evidence
and the theory about those evidence. Anti-matter is by definition
capable of real distruction if it would be combined with matter. So
how is it possible from this combination the creation of something as
powerful as the Gamma ray.
The problem of modern physics is the gap between experimental data and
theory suitable to explain it.

Note: Both have mass!

Thanks a lot