Oh, yeah, I perfectly understand (well, anyway as far as humans do) the expanding-metric nature of the universe, and the theoretic construct of "dark energy" driving it (although I find the latter "suspicious" and half-expect that theory to be eventually shown wrong or incomplete).

What I was questioning isn't really the expanding metric of the universe. What I was questioning was that starting point. At the instant of the Big Bang, people tend to think the universe was all located at a single infinitely-dense point, from which it's been expanding ever since. But it would look the same to us, if that initial point was merely one location in an infinitely-large infinitely-dense continuum, which has been expanding ever since. The lightspeed horizon prevents us from seeing the rest of it whether it's infinite, or just very very very large.

Our *observable* universe has a particular finite size. But the stars out on the edge of our observable universe, we think, have a different observable universe which includes a bunch of stuff we can never see. If we traveled there at lightspeed, we would find that that stuff had passed beyond their lightspeed horizon by the time we arrived.

The debate over how big that additional volume of the universe must be is ongoing, but as nearly as we can tell the metric of space is flat. Meaning, we're pretty sure it doesn't curve around on itself making a finite whole unless it's so incredibly vast that we can't even detect the "curve" that it would be wrapping around. And yet people think that having started from a point it must be finite.

My question is why we think it started from a point. "Infinite density" does not mean "Infinitely small." An expanding spacetime metric does not imply that the Big Bang (or the universe) is finite.

Maybe what's out there beyond the lightspeed boundary is just ... more universe, forever.