Before the Big Bang, there was space: Space consists of space quantum systems, which continually split, producing new space quantum systems. Real - as opposed to virtual - particles came into existence when there were sufficient space quantum systems within a sufficiently small volume that their combined mass-energy fluctuations contained sufficient (negative) gravitational energy to counterbalance the mass-energy from these particles, so that these fluctuations persisted. The resulting clump of particles produced an emission of radiation very similar to that predicted by the Big Bang.
The universe can only expand, not contract: The universe expands because quantum fluctuations continually produce new space quantum systems. It cannot contract, because there is no mechanism by means of which space quanta can be eliminated.
Gravitational potential energy and mass-energy are two sides of the same coin: The total gravitational potential energy at any point in space is physically present at that point. When particles produced by a quantum fluctuation persist, the magnitude of the gravitational potential energy increases in the same amount as the additional mass-energy, because they are two sides of the same coin. The gravitational potential energy produces time dilation with respect to the time dimension according to the General Relativistic equation for this dilation.
The universe is most probably the 3-surface of a 4-ball.