[-9156]()
Conservation of mass isn't actually true. It just appears to be true in most ordinary situations, since physical force chemical reactions aren't strong enough to create/destroy matter.
When Einstein said E=mc2, it meant that you can actually turn matter into energy and back. For example, when [antimatter](reddit.com/r/TeamAntimatter) and normal matter come into contact, they annihilate each other completely, and a bunch of light and super-fast neutrinos come out.
...Except conservation of energy isn't actually quite true either. It only holds on large scales because of the Heisenberg uncertainty principle. In order to know the exact energy of something (that is, to get ΔE = 0), you need Δt = infinity. If you only measure something for a finite amount of time, then you will have to have some uncertainty in the energy.
And energy conservation doesn't even make sense if your energy isn't a well-defined number. On very short times/distances, you can actually have random fluctuations in energy, just out in the middle of the void of space. Energy and light and particles constantly come into existence and disappear, with most of these events happening very quickly (<10-20 seconds, for example.)
So even in the absence of any energy/matter, you can still make it out of nowhere, for a very short amount of time. Conservation only holds on large scales.
...Except it doesn't hold on large scales either. General relativity throws that out the window too. As space expands, the light traveling through space loses energy and gets redshifted. It doesn't really go anywhere, it just sort of disappears.
tl;dr: Energy conservation is an absolute, thoroughly tested law of physics, that always holds unless you're dealing with small things, or big things, or fast things, or slow things, or screw it this law is pretty useless actually
