You misquote the first law of thermodynamics. It doesn't say that matter can't be created nor destroyed. It says that energy can neither be created nor destroyed, only converted from one form to another. Matter is a form of energy that is in a conserved state. Matter can still be converted back to energy and energy can be converted into matter, but the energy itself can never be created or destroyed. There is something called quantum fluctations where even in a perfect vaccum, tiny particles of matter are spontaneously created out of energy, then converted back to energy again, proving that matter can in fact be created and destroyed but never the energy itself.
