Does the big bang theory hold that there is a possibility that the whole of the observable universe came out from NOTHING? I mean if all of this universe was occupying an extremely small size of a Planck length, then it appears to be tempting that the universe before that was even smaller and smaller till we reach zero size. i.e. till we reach no universe. So the story of existence begins with nothing followed by the sudden emergence of the universe. Is that what the current big bang theory is claiming? Or is it an unsolved question?
Here is the current Big Bang model, a cut in two space and one time dimension
Note the fuzziness at the beginning , the Big Bang. Quantum mechanical behavior is introduced, so there is no longer a "point" for the Big Bang , but a probability locus, so no zero, as with all quantum mechanical states.
So it is not out of "nothing" but out of a probability locus.
Physicists expect that the underlying level of all classical theories is quantum mechanical, and an effective quantization of gravity is proposed, as gravity has not been definitively quantized yet, so it is still a matter of theoretical arguments .
Now if this probability locus is unique or if there are many universes ,again it is a matter of speculation.