@MrStickBall
Look up quantum fluctuations. The basic idea is that the positive energy that is expanding the universe is (almost) cancelled out by the negative gravitational energy contracting the universe. This makes the overall energy of the universe (almost) zero.
An equation derived from the Heisenberg Uncertainty Principle is: uncertainty in Energy * uncertainty in time >= hbar
Where hbar is Planck's constant diveded by 2pi.
This means that even something with zero energy (a perfect vacuum) has energy fluctuations, and therefore has mass fluctuations. The universe could be an energy fluctuation from a vacuum (i.e. nothing).
(Although this sounds outrageous the Heisenberg Uncertainty Principle is an absolute fact of the universe and it does allow for vacuum fluctuations (it happens billions of times a second all around us)).
