I guess the short answer to this is that we have no idea. We do not have any informed idea as to what existed before the moment of the Big Bang. Any thoughts about what existed before that are more the subject of religion and faith than they are of science.
Scientists believe that, at the moment of the Big Bang, all the matter and energy that now exists was contained in just one little point. Then that point essentially exploded and all the matter and energy came expanding out from it.
When you look at it like this, it was not a case of something coming from nothing. There was matter and energy there before the Big Bang. But where did the matter and energy come from? That's the part that no one can know.
According to basic principle of science, it is not possible to create anything out of nothing. But in spite of the popular belief science in not perfect. There are many exceptions to well established and accepted scientific rules. Also there are may phenomenon that are not understood or explainable by science. The Big Bang is a phenomenon that is among biggest mystery of the universe about which science has very little understanding. Science is able to logically reconstruct the sequence of events that took place at the time of what is called the Big Bang. But science has no proof to say with certainty that this is exactly the way things happened. And when it comes to explaining why the Big Bang happened or what was the source of the energy and matter that the Big Bang created, science has absolutely no answers. Science cannot even tell with any amount of certainty whether the energy or matter existed at all before the Big Bang. Thus science has no alternative but to accept the possibility that the Big Bang represents an exception to the general rule that nothing can be created out of nothing.