Matter gets created and destroyed all the time. Matter isn't even approximately conserved. If a particle and an antiparticle — which we will arbitrarily decree to be "matter" — interact, they're going to mutually annihilate and disappear forever: poof, gone. Something of them remains; we call that something a "conserved quantity" or "conserved charge." But matter itself is not conserved. It comes and goes as it pleases.

You were probably taught, contrary to matter, that energy is exactly conserved and that it can never be created or destroyed. Yeah, that's not really true either. Energy is in fact vanishing from the universe all around us. See, conservation laws exist only where there are exact symmetries of nature. Angular momentum, for example, is exactly conserved; that's a conservation law that we've never seen broken and that we don't ever expect to see broken. It arises from the symmetry of rotation in space. Because space is symmetric under rotation, angular momentum must be conserved.

This relationship between conservation laws and symmetries has a name: Noether's theorem. Emmy Noether worked it out mathematically; it's an absolute mathematical fact as plain and inescapable as the fact that two plus two makes four. Wherever a continuous symmetry exists in nature, there also must exist a corresponding conserved quantity that can never be created or destroyed but that must remain constant everywhere and forever.

So getting back to the energy thing: If energy were conserved, the corresponding symmetry would be the symmetry of translation through time. In other words, if it's possible to do an experiment on Monday and then exactly reproduce that experiment on Tuesday and get identical results — controlling for every single variable — then energy would be conserved.

But it's not. Possible. Because the universe is expanding.

The expansion of the metric is happening so slowly that we can't measure its effects between Monday and Tuesday. But if you did an appropriate experiment today and then tried to exactly reproduce it in a billion years, you'd get different results because the metric is a function of time. It isn't constant; its value depends on when you measure it. So the continuous symmetry of time translation that would be necessary for energy to be conserved … just isn't. Energy, it turns out, is not conserved on the scale of the universe.

We can even see this with our own eyes — or rather, can't see it with our own eyes. Billions of years ago the universe was filled with light: visible light, exactly like the light emitted by the sun. The whole universe, at that time, was filled with a hot plasma of mostly protons and electrons — a hydrogen plasma, in other words, with trace amounts of helium, lithium and beryllium. Protons and electrons are both electrically charged so they interacted with each other like crazy, although they had too much energy to settle down and make atoms with each other. So the whole universe was full of sunlight, essentially.

That light is still out there, filling all of space. But we can't see it because over billions of years the metric has expanded, stretching out the wavelengths of all the light in the universe. Each individual photon got stretched by the expanding metric; a 400 nm visible-light photon got stretched all the way out to a meter-long microwave photon. Which is why we can't see the light of creation with our eyes: We can't see microwaves. But if you point a microwave-sensitive telescope at the sky you find that the whole universe is still shining with the first light. It's just been dimmed by time and metric expansion.

Well, the energy of a photon is inversely proportional to its wavelength. Smaller photons have more energy than big ones. As those tiny visible-light photons got stretched out, they lost energy. Where did it go? Nowhere. It just disappeared. Vanished from the universe. Because energy, on the scale of the universe, is not conserved. The expansion of the metric means that there's no symmetry of time translation, which means that energy is not a conserved quantity. Energy can be — is constantly being, in the sky right over your head — destroyed.

But again, the metric is currently expanding so slowly we can't measure its effects on human scales. So we can get away with saying "Energy cannot be created or destroyed" if we stick a little asterisk on it and say "in the limit of weak fields and a metric that is close to being constant with respect to time." As long as our experiments are small enough and local enough, energy won't be destroyed much. It won't be destroyed enough for us to notice. We can pretend that it's not being destroyed.

But we still sometimes tell kids that "energy cannot be created or destroyed" without giving them the rest of the sentence, which is a shame, because the whole story is much more interesting than just blurting it out as if it's God's own revealed truth amen.