actually the big bang theory doesn't really explain how matter came to exist, it's just an explanation of how incredibly dense space began to expand outward and become the universe we actually observe now. we don't really know where matter and energy came from, and we don't claim to know. so as far as i'm concerned, god is just as good an explanation for that as any other
The big bang theory is just a comprehensive explanation of how the universe was from around 10
-43 seconds old to a few hundred thousand years old. It explains how matter (atoms, molecules) was formed, how energy (photons, other types of energy which i can't remember) behaved, how the fundamental forces (gravity, electromagnetism, strong/weak forces) behaved and affected the universe.
It does not claim to know how the energy that formed the matter came to be or why the big bang occurred, not that "why did the big bang happen" is even a question that makes sense in the first place.
What you're thinking of is called "The God of the Gaps." Its basic premise is "Science can't explain 100% it at the moment, therefore god did it." Which, from a scientific standpoint, is fundamentally flawed.