Law of conservation of energy is a concept in modern physics. It holds for time scales of the order of at most the age of the Solar system. When the Solar system is destroyed all “tangible matter” is destroyed down to the suddhashtaka stage.
We do not know (science does not know) much about what happens at long time scales. So, it is not possible to explain phenomena in long time scales using conservation of energy.
Any sankata is “borne” (past participle of “bear”) by “dhamma”, the kammic energy that created it. “Dhamma” means “to bear”. Everything is created by the mind and each such thing has a finite lifetime. In a way, that is related to the concept of anicca; see, “What Are Rūpa? (Relation to Nibbāna)“.
Some sankata disintegrate into other types within fairly short times. For example, a human body is decomposed into other inert matter, and those may break down eventually to electron/proton levels. But electrons/protons also have finite lifetimes even though very long. A suddhashtaka is the ultimate building block (much smaller than an electron/proton) and it has ultimate lifetimes of the order to half of a Maha kappa.
The point is that any sankata is destroyed after some time. We cannot explain these things with the limited vocabulary of modern science. Conservation of energy is such a concept in modern science. It will work well for science which deals with fairly short times scales, but NOT in the time samsaric time scale.