We Never Know
No Slack
That sort of nothing may not be possible. And that ais the point. When a physicist says that the universe could have come from nothing you would need to ask them how they define "nothing". As far as mass and energy it appears that the universe could have come from "nothing" because the total energy of the universe when measured is as close to zero as they can measure.
"May not be possible"?
If absolutely nothing ever existed, there would still be absolutely nothing. IMO I think its safe to say isnt possible