We seem to agree that our universe had a beginning. But I don't follow where you are going from here. Are you suggesting that the mass-energy that makes up our universe did NOT have a beginning, that it was eternal? If so, how do you reconcile this with our universe having a beginning? Or are you using conservation of mass-energy to reject the Big Bang?
The universe
as we know it had a beginning. But what I am stipulating is that it was not a 'poof' into existence scenario. It was more akin to a state change from one form to another.
The fallacy most are making when they examine the Big Bang is that they make an assumption that it was a something from nothing scenario. That is false. The Big Bang was actually an expansion event where space-time formed from the energy resident in the singularity. A good analogy is to consider states of matter: gases, liquids and solids. An ice cube 'came into existence' as an ice cube. But it originally began as liquid water. It changed states.
Note that this is in no way a 'rejection' of the Big Bang. The Law of Conservation of Mass/Energy follows along perfectly with the notion of the Big Bang. However, the Law of Conservation of Mass/Energy
is a refutation of the creation supposition, since it automatically refutes the notion of the universe having simply been created from scratch.
There are of course other avenues the creation argument can go. That god used part of his own 'energy' to form the universe. However, this does not seem to conform to scripture.