: It would weight exactly the same both before and after the explosion. Matter is not the same thing as mass and radiation is not the same thing as energy. Mass, universally, cannot be converted to energy.

: Mass, radiation, gravity, particles, fields, etc. are just different "looks" of energy. So, if electron and positron annihilate this means that about 1 Mev of energy in the form which is not moving (but say, oscillating) turned into 1 Mev of energy which is moving (gamma-radiation) This energy can turn back into electron-positron pair again when 2 or more gamma-photons collide. So what we called mass is just another appearance of the same energy. According to string theory it is just energy of oscillations of tiny loops of space itself.