And if no other alternative can be given, to a universe from nothing, than what is left?
The fact that
nobody currently knows why there is something rather than nothing! What's so hard about admitting that?
Is it fear that if people don't embrace Krauss or speculations like his, then theistic creationism will somehow have been justified? That's what you seem to be implying with your "then what is left?" and your occasional references to ID.
Imagine that we want to explain some mysterious phenomenon. And suppose that somebody proposes an explanatory hypothesis, let's call it A. Now suppose that A suffers from obvious defects, not least of them absence of supporting evidence, fundamental circularity and ambiguous and inconsistent use of important terms.
What should we do? Just ignore the difficulties, accepting and believing A because it's the only hypothesis that we currently have? That seems to be your argument. (Turtles, all the way down.)
Or should we acknowledge that we don't currently have any satisfactory hypotheses, try to learn from our experience with A and try to invent a better hypothesis that doesn't suffer from the kind of defects that A suffered from?
Explanatory hypotheses proceed from initial assumptions. But in the case of the 'something from nothing' problem, if we are starting with
nothing in the strongest sense of total nonexistence, then we have ruled out any initial assumptions. Reality is just popping out of nowhere (literally). So not only will the theorist have to invent new hypotheses, he or she will have to invent a whole new way of reasoning in theoretical physics that doesn't assume the truth of quantum field theories or any other beliefs of physics. He or she will have to start with no initial assumptions at all, not even logical or mathematical ones.
I don't see how it can be done. That's why I suspect that this is a problem that science will never be able to answer.
A scientific answer to the 'something from nothing' question seems to me to violate science's methodological naturalism. Science concerns itself with what happens
within the world of nature. It addresses the contents of the natural world and how those contents interact. (Often trying to give mathematical formulations for those interactions.) The problem of why there is a world of nature at all doesn't seem to be that kind of problem. That new question addresses the world of nature's relationship to something else, even if we call it 'nothing'. (Calling it 'nothing' just puts extremely severe constraints on how we can conceive of it.) That's why this is a metaphysical rather than a physical problem.