Do you believe that the Big Bang started the entire universe or just our planet? And if so where did the material of the two asteroids/ rocks/ past planets come from? Do you personally believe the Big Bang theory?
The Big Bang was the beginning of our observable Universe, where it became filled with matter and radiation, and where it began cooling while it expanding, for the first time.
Our planet did not form until many billions of years (roughly, 9 billion) after the Big Bang, and it took many generations of stars to live-and-die before it brought us there.