For the record, planets aren't moving away from each other due to universe expansion. Galaxies and the groups they comprise, yes. But the motions of planets are more governed by the loss of mass of the primary they orbit, and/or the gravitational intrusion of other stars that pass nearby.They've calculated the rate at which planets and other objects in space are moving away from each other, and noticed the cosmic background radiation, which is evidence of the universe expanding, and evidence for the big bang. Just because we can't rule out several possibilities for what happened before the big bang does not mean the big bang could not have happened. It fits within the theories and the properties of the universe as we know them.
"What happened before the big bang" is a useless question, except to prove the ignorance of the questioner. They don't understand the concept of space-time if they're asking what happened before time began. It's like asking what is north of the north pole. People with half a brain should discard as useless garbage the opinions of those who think that our inability to say what is north of the north pole is some sort of proof of god's existence.