Einstein's mass energy equivalence

Mass is a packed form of energy (as $E=mc^2$) suggests. Also we can convert mass energy into another forms such as in nuclear reactions. Does the mass energy remains packed until it is urged to get converted? Or is they are converting into one another such that an equilibrium has established?(just like dissolution and crystallization of sugar in water.)

Answer

$E=mc^2$ expresses an equivalence between mass and energy. It says nothing about how, when, or why one form of mass/energy is converted to another. For example it provides no reason for why, when two protons with enough KE collide, three protons and an antiproton may result. It just confirms that it's energetically possible.