equivalence between mass and energy
Already in special relativity, it turns out that (relativistic) mass and energy are really no more than two different ways of looking at one and the same physical quantity. To every form of energy, there corresponds a mass – if you heat a body up, increasing its thermal energy, you automatically increase its mass. On the other hand, simply because of the mass of its constituent particles, every chunk of matter contains lots and lots of energy. In situations like the annihilation of particles in contact with their anti-particles, resulting in electromagnetic radiation, this matter-energy can be transformed completely into more ordinary types of energy.
The formula relating a mass to the equivalent energy is Einstein’s famous
E=mc² (“E equals m-c-squared”)