By most criteria, we haven't got an explanation-space, let alone an explanation, for why anything exists. We can't imagine what an answer to the question would even look like. It's apt to seem so astonishing that there should be anything at all that everything else about the world can - if one is of a philosophical cast of mind - appear as trivial detail in comparison. It is not clear whether this fundamental mystery of existence will ever be solved. For it is normally reckoned we don't know how, and with what conceptual resources, we might begin to start looking for an answer. We don't even know here what the amorphous notion of an "explanation" would amount to.
Much ado about Nothing?"Nothing comes from nothing"
(LUCRETIUS : The Nature of Things)
On the other hand, misplaced optimism that a solution can be found isn't uncommon. Sometimes such optimism derives from verbal manoeuvres aimed at damning the whole question as meaningless. Sometimes pseudo-solutions derive from a confusion with deep but conceptually distinct puzzles such as: why there is [currently] a preponderance of matter over antimatter? Or what if anything came "before" the Big Bang? The latter problem at least "feels" deeper than the origin of matter. Yet it still doesn't address the fundamental enigma. Even if a Hawking-inspired "no boundaries" conjecture is correct, and there is no "before", the central problem of why anything exists remains as intractable as ever.
There is another formidable problem. We don't have any clear idea how to block the vicious regress of explanation, whether causal or logical, facing various otherwise conceivable prospective candidates we might one day arrive at. For instance, many cosmologists have speculated that our universe is the product of a spontaneous fluctuation of the quantum vacuum (again, net energy = 0, insofar as the net energy of the universe can be well-defined). Subsequent symmetry-breaking phase-transitions in this very early post-Planckian era triggered off a brief period of exponential inflation. Its effects account for the large-scale properties of the "visible" universe today. Inflationary scenarios are certainly intriguing. Indeed, there are other models, such as variants on Linde's eternal chaotic inflation, in which elsewhere - beyond our parochial little multiverse - there is nothing to stop inflation ever ending. Inflation without end is an utterly dizzying prospect. None of these options as they stand, however, offer any obvious prospect of blocking the explanatory regress. Even so, scientific inquiry still seems more likely to yield fruitful answers than armchair metaphysics. For if we could ever come fully to understand the properties of what fundamentally exists, and unravel its causal history, or perhaps show how space-time emerged from something more primitive, then we might have a better chance of coming to understand why it - or indeed anything - exists too.
Physicists sometimes say that all the really interesting things in the world happened during the first second. Today many theorists think we understand most of that second rather well. Yet we have no idea why the underlying reality of the world is, say, a rich quantum vacuum(?)(?) (or stringy vibrations, branes, etc.). This ignorance persists even though we now fondly suppose that we know some of a quantum vacuum's properties. So unless all forms of meaning holism are false, then perhaps we won't really understand the QM vacuum's properties at all, or anything else either, until we understand why, in some sense, the quantum vacuum and its metastable states exists. Worse, if the history of science and philosophy is any guide, then the ultimate answer to the mystery of mysteries may depend on a radical revision of our concept of what it is to be an explanation, too. Yet this revision wouldn't be available until we were well on the way towards knowing the solution on which it depended. So can we "bootstrap" our way to an answer? Let's hope so.
These reflections may seem unduly pessimistic. Yet scientific rationalism and the whole epistemic enterprise depend, not just on (at least a large methodological measure of) physical reductionism, but in practice on a large measure of semantic atomism too. This is so in the sense that they presuppose that our ignorance of the solution to such mysteries as the nature of consciousness, and an explanation for existence itself, doesn't systematically infect and stultify the conceptual content of everything else we think we know; and that the solution to these mysteries wouldn't revolutionise or render obsolete the semantic significance of all our previous terms.
Yet perhaps such semantic quarantine isn't feasible. Our position on learning the ultimate answers might be like that of a religious devotee whose loss of faith transforms his conceptual scheme though he retains its lexical shell; or like that of a rationalist philosopher who thinks he understands consciousness prior to expanding and radically re-ordering his semantic and evidential base with psychedelic drugs. Or, possibly more relevantly, since we don't have even a conceptual shell to be getting by with in the case of explaining the world itself, perhaps it's like trying to understand life without any understanding at all of the principles of Darwinian evolution. Such a feat simply isn't possible.
Admittedly, and in an unknown way, the question of why anything exists is probably ill-posed. The question may reflect some erroneous Collingwood-style Absolute Presupposition or background assumption that humans can't access. Or alternatively, its flaw may lie in a (false) proposition that is too trivially and glaringly obvious (to us) to appear worth formally articulating and defending. Yet it would seem too facile to write off the Fundamental Question, and any better formulated successor, as cognitively meaningless without some no-less-deep explanation of why this potential failure of reference should be so likely a prospect.
Treating the question of why anything exists seriously is not the same as saying that ultimately anything really could, in some obscure, non-epistemic sense, be otherwise than it actually is. Asking why there is Something rather than nothing at all isn't saying that non-existence was a substantive(!)(?) possibility. Indeed an implication of the zero ontology to be argued here is that the existence of anything else other than what exists is - if what does exist is properly understood - logically incoherent, including the notion of unrealised ontic possibility itself. For perhaps in all but a heuristic sense there is no difference between x and necessarily x. Given this is the case, then the notion of real contingency turns on a psychologistic misconception of the link between possibility and the imagination, because everything must be exactly how it is on pain of our lapsing into incoherence. In the case of the notion of nothing at all ever having existed when construed as a real possibility, then even the link with imagination breaks down. This is because one can't imagine nothing whatsoever existing. One always imagines some property or other.
But scepticism about real contingency isn't in itself very helpful to solving the riddle of existence. For one will still want to know why things can't be otherwise. A working assumption in this context is that the ascription of possibility is always relative to background conditions. If the background condition is simply everything [net = 0??] that (tenselessly) exists, then the possibility of it being otherwise in anything but the epistemic sense is incoherent. The task is to explain why it is incoherent. Such modal scepticism might seem incompatible with the Everett interpretation - superficially the fan of modal contingency's ultimate free lunch. Yet all the nominal Everett "branch" worlds are equally real. Moreover, properly understood, there is only one world; it's just that most of its classically inequivalent coarse-grained "branches" don't interfere with each other very much. [This is perhaps simplistic. There may in fact be (cf Lee Smolin) a (pseudo?)-infinite population of zero-rated multiverses(!) whose evolution and complex interconnection via black-hole "umbilical cords" can be described by Darwinian principles. "Hairy"? I'm betting not]
Unfortunately, one can often pose the right questions embodying the right concepts only when one already glimpses the answers. Here we don't have them. So these notes really amount to a hunt for an explanation-space, not an offer of an explanation. The woolliness of the proposed answer is redeemed only because the zero ontology propounded here illuminates how the ultimate explanation might in principle be exact, rigorous, predictive and exhaustive i.e. not wordy and "philosophical" at all.