creatio ex nihilo“At first there was nothing, then out of nothing the Big Bang happened and the universe began” is a popular misconception of the Big Bang theory. It’s completely wrong.
The Big Bang theory does not attempt to explain what things were like “before” the Big Bang. It’s not a theory of creation at all! It only offers a model starting with the inflationary epoch, when the universe was already expanding rapidly.
Prior to that, our existing theories break down and we need a different physical theory (commonly called the Grand Unified Theory or GUT). We don’t have it so we can go no further back.
The point is: back in the inflationary epoch the universe was all there, it was just much smaller. The point of the theory is to try to explain the extreme uniformity (isotropy) in the distribution of galaxies, gas, dust, and light we observe in the universe today (among other things, such as dark matter). It’s a much better model in terms of its simplicity and predictive power than any steady-state model of the universe that has been proposed.
But it’s got nothing to do with creation!