It has always been here. The Big Bang is not a singular event.
The matter in the universe is constant. Gravity makes matter attract matter. The greater the density the greater the gravitational pull.
So the first part of the Big Bang is matter being attracted to a point. As more matter is added the attraction becomes stronger.
Eventually, the matter is gathered into a huge star. Which then collapses followed by an explosion the likes of which we have never seen. The matter is hurled outward. At some point, the matter slows and is drawn back. Repeat as necessary.