I'm pretending you actually want the answer. Probably somebody does, at least.

People naively think mass means "stuff" and energy means, I don't know, "wobbly stuff." And they think conservation of energy is something to be taken for granted, as an obvious, common-sense fact. But as we've learned time after time after time in physics, whenever you dig deep enough, your common sense is eventually wrong.

In physics, mass and energy refer to very specific things, and their conservation follows from certain underlying assumptions. These assumptions hold accurately in your and my limited range of experience, but these assumptions are strongly violated in the early evolution of the universe (or in many other extreme astrophysical events).

Specifically, energy is defined as a "charge" (in a technical mathematical sense) associated with "time-translation symmetry" of the universe. "Time translation symmetry" is the assumption that the underlying fabric of spacetime geometry is unchanging with respect to time. If that assumption holds, then one can show that energy is constant. (Mass, by the way, is related to energy, but it would take us down too technical a road to clarify the relationship in general. At the very least, we've all heard of E=mc^2.)

So when the universe is not appreciably changing in time, then energy is conserved. When the universe is appreciably changing, then energy is not conserved. In the time shortly after the big bang, the universe was obviously changing very rapidly.

Energy is "mostly" (though not exactly) conserved in the universe as a whole today (as long as we leave dark energy out of the picture, which is totally not conserved). But in the early universe it wasn't.

This is all standard physics that has existed for a hundred years now. Einstein's general theory of relativity. You might be surprised to hear that the general "theory" of relativity is more fundamental and more accurate than the first "law" of thermodynamics. But as you've been told hundreds of times around here, those words don't mean what you think they mean.