What came before the big bang and other questions physics can't answer.... yet. New Scientist P30-31

Pause. Rewind. Suddenly the outward rush of 200 billion galaxies slips into reverse. Instead of expanding at pace, the universe is now imploding like a deflating balloon: faster and faster, smaller and smaller, everything hurtling together until the entire cosmos is squeezed into an inconceivably hot, dense pinprick. Then pshhht! The screen goes dead. According to big bang theory – our best explanation for why space is expanding – everything exploded from nothing about 13.8 billion years ago. Cosmologists have been able to wind things back to within a tiny fraction of a second of this moment. But now they’re stuck. The trouble is, our understanding of space-time, and gravity in particular, is built from Einstein’s equations of general relativity, whereas the extreme conditions of the very early universe can only be described by quantum mechanics. No one knows how to reconcile the two to take us further back. 'The rules we have simply don’t work in that regime,' says Professor Carlo Contaldi at Imperial College London. 'Nothing makes sense any more.

Text taken from New Scientist P30-31