Reply #15: OK since Wikipedia explains it better than I do

[tocqueville]

"A gravitational singularity occurs when an astrophysical model, typically based on general relativity, predicts some type of pathological behavior of space-time, such as a point of infinite space-time curvature. The term is closely related to the mathematical notion of "singularity": a gravitational singularity occurs when the equations produce a mathematical singularity."

"The Big Bang cosmological model of the universe contains a gravitational singularity at the start of time (t=0). At the "Big Bang Singularity," the model predicts that the density of the universe and the curvature of space-time are paradoxically infinite."

in other words before "it started" mathematical and physical models show that the universe was infinetely dense and the curvature infinite (which is for physical reasons a paradox). Why it exploded or changed state, we don't know, because there are no ways of "measuring" it today.

Physics never say that the universe came out of "nothing". We cannot explain the something before the something, because the something before is fundamentally different from what we can understand today.

http://en.wikipedia.org/wiki/Big_Bang