This is a question I get asked a lot as an atheist. And it’s a terrible question for two reasons. The first is that is presupposes that nothing is a possibility, that it is an option. Asking “why is there something rather than nothing?” only makes sense if nothing is a reasonable, possible alternative to something. But we have no examples of nothing to point to to demonstrate that it was ever an alternative, that it could have been possible for there to be nothing.
Even if we accept that nothing was ever a possibility, the second problem is that this question posits that nothing is not only possible, but also more likely than something. After all, if the questioner accepted that something was as or more likely than nothing, the question wouldn’t need to be asked (why is there x rather than y, when x is more likely or as likely as y, is always answered with “because x is as likely or more likely than y.”) So the question includes this other hidden hypothesis, that somehow nothing is more likely to have been, and the fact that there is something is somehow unexpected, unusual, or surprising.
But suppose that there were nothing. No space, no time, not even any sort of quantum vacuum a la Lawrence Krauss. In the absence of any stuff, in the absence of any laws of physics, of quantum mechanics, there are no restrictions on what can or cannot exist. In other words, if there is nothing, then there is also nothing to restrict the likelihood of or prevent the existence of something. The rest is mere probability: if there were nothing, then continuing to be nothing is one possibility, our universe or any other single universe beginning to exist is another, a multiverse with k universes is another, etc.
In other words, if there ever was nothing, it would be equally likely for there to continue to be nothing as it would be for our universe to exist. What is even more astonishing is that it is in fact more likely for a multitude of universe to exist that for nothing to continue being nothing or for one universe to exist (it is equally likely, for example, for there to be three universes as two universes as one as nothing, but more likely for there to be two or three universes than one or nothing.)
So, in fact, the answer to the question “why is there something rather than nothing?” is that, if there ever were nothing, it would be far more likely to cease being nothing and for many universes to exist than for nothing to continue being nothing.