Welllll....Rocks were not involved. You can't think of the Big Bang as two large boulders colliding and then fragmenting into little stars and galaxies. Our observations tell us that the universe was once much younger, hotter and denser. We measure a feeble 'cosmic background radiation' and see that it has a temperature of about 2.7 degrees Kelvin, but the universe is expanding now, so if you follow it back into time, the cosmic background steadily becomes hotter and hotter until by a few minutes after the Big Bang, it's temperature is in the billions of degrees. There is no 'cold vacuum' left in space. Instead, it is a seathing, dense plasma of charged matter and gamma ray light, and every corner of the universe is as thick as the Earth's atmosphere with this lethal plasma. At still earlier times, Big Bang theory PREDICTS more of the same, with increasing temperature and density, until we reach the mysterious 'Planck Era' where only the bravest of physicists feel secure about what they say, in light of absolutely NO data to dispute them!
What happened 'before' the Planck Era to start the whole shebang going is not known based on what we know about physics; but it sure as heck didn't have anything to do with rocks slamming together! Rocks didn't 'evolve' from matter until 1 billion years after the Big Bang!