A mass is another form of energy. When a mass ceases to exist as 'matter', it exists as energy - in the forms of energy we generally know (light, heat). But is this so simple? When a mass exists in its usual form (a particle for instance), it creates a space time curvature. When the mass ceases to exist anymore in its usual, can it be not that that the curved space time returns to its initial state with the release of energy, - the energy that we actually obtain?
Taking an analogy to make the question clear --- When an object is placed on a stretched string, the string gets deformed storing some energy in it. When the particle is taken away, the string gets un-stretched with the releases of the energy. Can such an explanation not run for mass energy conversion?
I am a beginner enthusiastic in cosmology, please clarify to me.