I know there is a lot of misconception amongst the general public involving the big bang theory. Most people outside the scientific community still believe that the big bang theory says that the all the universe - and not just the observable one - started from a single point, and that the beginning of the universe was something like an explosion rather than an expasion. I am lay in this affair, but I've been doing some kind of research in order to get a better understanding about what this theory really tells us about our universe. I mean - and correct me if I'm wrong - we know that, in the past, the universe was much more hot and dense, and if we extrapolate this for a very distant past, we get a singularity. Science doesn't know what happens there, so the big bang theory just tells us about things that happenned after this singularity, right? My doubt is: we know that the universe is something like 13.8 billion years old. But this is the time elapsed since when? The singularity, that we don't know nothing about, or the beginning of the expansion? And this is the age of the observable universe, or the whole one?