If the big bang expanded the universe from a singularity point, that means that there was matter before, so where and how did matter form for the first time if it's even possible to know such a thing?
"before" is not the right word. It suggests that there was an arrow of time which allowed to trace events. But time was created only with the big bang, there was no before.
We don't know where the matter came from, or what caused the Big Bang. One hypothesis suggests that it was caused by the collision of branes in other universes, but there's no way to verify this, so in a strict sense it isn't science. Many researchers think we'll never have an answer this.