Most likely, it will be one of those things that scientists never know. But then that is the case a lot more often than you may think. After all, know is a pretty strong word. The things we really know are the results of our observation. I know what I see in a telescope, but then I interpret what I see (what I know) in terms of some model, or hypothesis or theory, and my interpretation is really not knowledge so much as it is a really smart guess (since I am talking about myself, I reserve the right to insist that all of my guesses are really smart ).

We know that we measure redshifts of galaxy spectra, and we interpret the redshifts in terms of an expanding universe. So I would not go so far as to say that we know that big bang cosmology, or the cosmology of the expanding universe is really true. But we can, and do know that the "predictions" from the cosmological models are consistent with what we know through observations. So we allow ourselves the luxury of calling it "right" or "correct", because in the practical sense of things, it actually works. And nobody has really come up with a better idea anyway.

With all that in mind, yes we can make cosmological models that tell us where the energy came from. It's called "pre big bang cosmology" and is mostly based on string theory, though there are more "ordinary" forms of quantum gravity that serve the same purpose. But it's all really just a lot of educated guesswork, nothing anyone can seriously claim to know.

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