From a [wikipedia article](http://en.m.wikipedia.org/wiki/Conservation_of_energy):

In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither created nor destroyed, but can change form; for instance, chemical energy can be converted to kinetic energy.  

What is the reason behind this Law: is there a proof that we can't destroy energy? I mean if we couldn't destroy energy in our universe maybe we can do it in other universes the point is why is it a law and not a theory