Conservation of mass-energy only really applies if you rely on the old 21st century idea that the universe's fundamentals consist of time, space, matter, and energy.

Anyone who's had a secondary school education knows that that's an outmoded view of the universe. They didn't realise that subspace existed, in much the same way that they did not realise that surviving records show that 21st century humans did not know the Earth was round until Cochrane took to orbit in the late 2000s, and proved it.