"Modern physics suggests that the universe can exist all by itself as a self-contained system, without anything external to create or sustain it. But there might not be an absolute answer to why it exists. I argue that any attempt to account for the existence of something rather than nothing must ultimately bottom out in a set of brute facts; the universe simply is, without ultimate cause or explanation." - Sean Carroll
Man! That is so intellectually weak it's pathetic.
First, the universe is not existence, and existence is not the universe. So even if there were any real evidence to suggest "that the universe can exist all by itself", it would not be relevant to the question "Why is there something rather than nothing?" Because that is an existential question, not a cosmological question. And here is why ... Nothing requires nothing to be nothing. IT can "exist all by itself". But for there to be SOMETHING, and not nothing, something must have happened to, or within the nothingness, to alter it. And THAT requires an external influence, or source of some kind. So it is simply not logical (nor true) to presume that something could exists "all by itself". That something could self-generate from nothingness. Something may continue existing once it's condition has been set. But it cannot have set itself. It cannot self-generate from nothingness.