Which "stuff" do you mean? The idea of the BB theory, or the matter of the universe? The idea comes from trying to explain various data, but I personally think the BB is the wrong explanation for it... there's more than one way to get the same answers.
If you mean the matter that exists in the universe, well, the BB theory doesn't explain where it all comes from either, just that it was at some point in time very condensed.
I think that all the "stuff" in the universe has simply always existed, always has, always will. It may change on a regular basis, but there never was nor will there ever be actual "nothingness." We like to think that everything has a beginning and an end, as that's how things "appear" to be on Earth. But if you think about it, nothing is actually created. Everything that is born or grown is just a reorganization of particular proteins readily available in the world around us, proteins that have always been around. And different proteins are reorganizations of atoms, and different atoms are reoganizations of protons, neutrons, and electrons, etc. Everything is just a reorganization of something else. That's what I think the universe as a whole is. Nothing is really "new" or "created", just reorganized. The universe didn't come from anywhere or anything. It has always just "been," and will always continue to "be."
