Since (x, y, z) is the singularity or point in Space at which all matter would disappear were the Big Bang expansion reversed and Energy requires nor Space and does not exist in Time, is there a repository for ebergy in some dimension still unknown to us?
Your premises are incorrect. There is no point in space "at which all matter would disappear were the Big Bang expansion reverse" - the expansion is the expansion of space itself.
All of space is expanding, so there's no one place we can point to and say "Aha! It's all moving to
this point!". Space expands like a balloon, with each point receding from every other point, but no point being the source of expansion. Everywhere sees everywhere else receding from it.
Because the Law of Conservation of Matter/Energy tells us that this Universe can not disappear materially unless it transmutes back, into an equivalency of Energy, isn't aenergy The First Cause, even "God?"
No. First, conservation laws are, ultimately, just theories. We say that energy cannot be created or destroyed because we've never seen it, and everything we've observed tells us that this law is true. But, nonetheless, it's still possible that the law can be broken, if only in unusual situations. This has happen with other conservation laws: Parity was once thought to be conserved, but the so-called 'weak interaction' violates parity. It's unusual, but it
does happen. There's no reason why the same couldn't happen with the conservation of energy.
Second, even if this conservation law were true, it only say that the
net energy is constant. A positive hunk of energy that's created at the same time as a negative hunk of energy, would not violate the conservation law, as the
net energy hasn't changed. This is how neutrons can decay into a proton and an electron (and an antineutrino) - though the proton and electron have electrical charges, their
net charge is zero, thus conserving electromagnetic charge, as per the law of the conservation of charge. In other words, the origin of the universe may well be the spontaneous creation of a finite hunk of 'positive' energy, that occurred at the same time as the creation of an equal amount of 'negative' energy (perhaps gravitational potential).
Third, 'God' is a very specific concept, and nothing is achieved by saying "I don't know the origin of X, but whatever that origin is, I will call it 'God'". The problem with doing that is that you then end up associating the usual traits of 'God' (answering prayers, healing the faithful, etc) to this hypothetical origin, which is silly.