This is the question that Leibnitz asked. The usual answer is that if there were nothing there would be no one to ask the question since we ourselves are made up of this « something ».
The only possibility would be that this question would be posed by an « immanent » being, independent of any material structure composing it, about this « something », which would be external to it!
In this regard, let us recall that, when we consider a spacetime, such as the one modeling our universe, we have argued that the concept of creation refers, implicitly, to an event located in time, (moment), and in space, (place), which would be, then, some pre-existing « physical » structures.
It is, therefore, inappropriate to attribute a « creation » to a spacetime, since it is a physical structure more synthetic than time and space which are of a non-physical character, resulting from an arbitrary internal foliation of its spacetime structure.
Indeed, not only is it difficult to see how it could be contained in a more rudimentary structure than its own, but above all it has no need for it.
This is why in the various articles and pages of this site we have refuted the term creation of a spacetime.
We have preferred instead to rely on the concept of « existence » in the sense in which « existentialist » philosophy defines it. Existence is something that can be observed but not deduced.
The term existence, itself, has, in spite of everything, retained an implicit space and timel connotation, because, even if we cannot deduce it, we could imagine a possible « creation » of existence, it would then be preferable to use the term « immanence » for a spacetime, a term that is free of any constraint of this type.
Immanence is then defined as the presence of something independent of any other consideration, such as the divine.