[wtcactus]

The accepted theory is that it started from a singularity. And time has no meaning when there's no space (i.e. at a singularity).

[gizmo686]

I'm not sure "accepted" is the correct word. It is a feature of our current theory, but we know that theory is incomplete. In particular high density regions such as the very early Universe are where we know ate theories start to break down, as the conflicts between quantum physics and gravity become relevent.

There are proposals for time stretching infinitely back, but we have almost no way of testing them.