Mass is a characteristic of matter and I remember being told, sneeringly, "The word is mass, boy, not matter" when being shown Einstein's E=MC².

I was also told that matter and energy were the same thing and that it was possible to destroy matter and release energy.  Fine, but I see no reason to assume that mass, a characteristic of matter, should vanish when matter is converted. To be sure, it would be a little tricky to measure, but if the energy released did have the same mass as the matter which was converted, it would explain a few anomalies, including 'dark matter'.