The answer to that is that the current model of the big bang theory has never postulated that the universe come from “nothing”.
You are still making claims about the BB that the BB astrophysicists never claimed.
All it does say is that the universe was almost infinitely hot & extremely dense - what they called the "singularity".
True nothingness cannot exist in the Universe, not even at this singularity. Absolute nothingness required there to be not only no matters (eg atoms) and particles, but also no energy, no fields of any kind, and not even space.
So if there are heat and energy in the singularity, then logically something still exist at the very instance of expansion.
Yes, I know that some theoretical physicists have postulate nothingness, like Lawrence Krauss. But you must remember that his book - A Universe From Nothing - is merely a theoretical speculation, it isn't even a hypothesis, as a hypothesis must have predictions that are potentially testable, which Krauss has not done in his book. Krauss is merely spinning his thoughts in the book. At best, it is a possible theoretical model, at worse, his concept is merely speculations, no better than unsubstantiated philosophy.
But going back to the nothingness. Even experiments have already demonstrated, where they have almost complete vacuum by removing matters and particles, and yet based on what Quantum Field Theory proposed, the vacuum is not completely empty, because the quantum fields still permeate the entire universe, including in the vacuum of space, and so energy still exist in the vacuum.