I will first comment on Rose Dale's leading entry. I consider this topic to be poorly worded. It is not a criticism of the article, since the topic is typically worded in exactly those words.
Although Heidegger described this as the fundamental question of metaphysics, the answer is quite straightforward at its base, if we are strictly examining a comparison between something and nothing. There is something because there is literally no such thing as nothing (at all), and there possibly never was. Spinoza and Einstein, among many other great thinkers, subscribed to this view that it is impossible for there to be nothing. Nothing is only ever the absence of something in particular, but it is never truly no-thing, since the very label ‘nothing’ implies ‘something’.
The opposite of 'something' is not-something, or not anything. This is different than 'nothing'. Yes, there is literally no such thing as nothing, which seems more analogous to an empty set of what
is, leaving still the set itself. Does not-anything need distinction? Distinction is needed for existence, but no claim of existence is made in not-anything, so not-anything seems consistent. A
state of not-anything is not consistent since the existence of a state is implied, so one cannot ask why the state is existence of anything rather than not. Careful wording seems to be the order of the day.
As a relativist, I must interpret the question differently. There being something relative to me does not imply objective being. Under pure relativism, there is no concept of objective anything, and thus any statement of the form "there is X" (even if X is 'nothing') is meaningless, not even false.
So my take on the question is one of meaninglessness unless I interpret it in some relative way like 'to me', but not necessarily any particular thing. Idealism is a tiny subset of relativism. Things can be relative to a rock, to a reference frame, or to an integer.