Something can't be created from nothing
How could you know that? We've never observed this 'nothing' (lets give it a loose definition of any potential material outside the boundaries of the universe) and don't have the first clue what properties it might have. I think we'd have to understand what's outside the universe, or what was there "before" it (before is a function of time, and time may not exist outside the universe for there to have been a before), to be able to answer whether the conditions were in place for the big bang (or any alternate explanation) could have occured.
It probably is unknowable but the scientific method doesn't circle around what we
know, it focuses on what we can
observe. It's then down to the eggheads and boffins to offer up hypotheses for these observations, and then develop experiments to prove themselves wrong. Once enough people have failed to falsify a specific hypothesis then it starts to become accepted, unless a competing hypothesis fits the facts better.
So if our observations are that every point in the observable universe is moving away from each other, then we can speculate that the whole thing is expanding to cover ever greater areas, with the implication it was at one point infinitely small and does currently have an 'edge' that is the outer boundary of the expanding area.
Whole thing could be wrong of course, but we'd probably need to be 'out there' to get much more evidence than we currently have, which doesn't seem to be on the cards (damn you 80s scifi, I really believed we'd have a moonbase by now).