Energy
In the early 20th century, scientist found that matter itself can be created from energy (energy and matter are interchangeable, in spite of everyday experiences).  This is just another change of form.  After these discoveries, the conservation law of energy was extended to become the conservation law of matter and energy: matter and energy can neither be created from nothing nor destroyed to the point of complete erasure from reality. Albert Einstein was the first to mathematically show this. (E = mc2)