Albert Einstein developed a theory about the relationship of mass and energy.
The formula, E=mc2, is probably the most famous one known from Einstein's special theory of relativity.
In essence, it means mass is just one form of energy.
Since the speed of light squared is an enormous number = 186,000 miles per second
a small amount of mass can be converted to a phenomenal amount of energy.
Or
If there's a lot of energy available, some energy can be converted to mass and a new particle can be created.