... "The law of conservation of matter or principle of matter conservation states that the mass of an object or collection of objects never changes over time, no matter how the constituent parts rearrange themselves.
The mass can neither be created nor destroyed."
That's from classical physics. You must account today for general relativity and quantum physics, resulting a the e=mc^2 equation and hold that the sum of mass and energy are thus related and bound in a closed system.
In the beginning there was no energy, no mass, no matter etc. Where did all this energy come from?
This is not what the "Big Bang theory" asserts.
https://en.wikipedia.org/wiki/Chronology_of_the_universe#Planck_epochRather it considers there to be a transition from such things being "undefined" to their being "defined as energy, mass, etc."
"Undefined" is not "no energy, no mass." It is different than say the quantity 0. "Time" does not exist either, so there is no "before" and no "after."
Science does not have questions to ask of "undefined." As an example, take the case where the universe has shrunk to a single point, a singularity, and ask, "How big is that point?"
That is nonsense, of course. Science would ignore that issue (although as I have noted the words used in your question formulate a cause and effect scenario that has no meaning when "undefined" is taken into account. Thus, science may influence religious thought by improving phraseology, but excepting that it will not address the question you pose of "original causation."