According to mass-energy equivalence, all objects having mass have corresponding intrinsic energy, even when they are stationary.
The “mass-energy equivalence” principle states that the mass of a system and its energy are the same property in any physical system.
This means that anything having mass has an equivalent amount of energy and vice versa.
According to this equivalence, mass and energy are inter-convertible. It means that if an object has mass, it has lots of energy just sitting inside. How much energy? This is given in Albert Einstein’s famous relation
E=mc^2
Where, m is the mass in kilograms, and c is the velocity of light in a vacuum. (c \approx 3 \times 10^8\rm \ m/s)
Since the speed of light (c) is a large number in everyday units the relation between energy and mass implies that even a small amount of rest mass corresponds to an enormous amount of energy, which is independent of the composition of the matter.