There is no reason to suppose that nothingness is more fundamental than being. In the Hegelian system, being and nothingness are unified in becoming. In a sense, neither being nor nothingness is the condition for the possibility of the other as they merely express different aspects of one and the same absolute identity. Dialectically understood, we would say that being (the whole or single) is becoming (the universal) in-and-through nothingness (the particular). In other words, being is a kind of irreducible subject that essentially destroys itself in the process of creating existence.
Note, however, that there is no inherent notion of space or time in this picture. A process does not require a concrete manifestation where that process is immediate, or perfectly simultaneous. That is, these categories could exist in-and-for themselves without reference to conventional notions of dimensionality, as might be the case with an unconditional first principle or necessary being. What we would have then is an eternalist conception of transcendental reality that would, in its own self, be the absolute ground for the immanent spatio-temporal development of the universe. Notions of "boundary" could, in the same way, be purely logical or mathematical existents.
This perspective was shared to different extents by the likes of Plato, Aristotle, and Spinoza, but it was most explicitly and convincingly demonstrated by Hegel. When I think about these issues, it is this view that has resonated most deeply with me and, in very broad outlines, it is the one that has seemed the most plausible. In contrast, I think the idea of the universe merely springing into being ex nihilo is the limit case of absurdity. It makes me very uneasy and I find it explains nothing about the pre-existent conditions which made such an uncanny singularity possible in the first place.
This is not the whole story, of course. None of the above really gets at the problem of causality or answers the question about why there might be a Platonic realm beyond the appearances. In this respect, I've found Terence McKenna to be the most illuminating. While he's often portrayed more as an esoteric mystic than a philosopher, I do believe he's on to something when he speaks about the "transcendental attractor at the end of time". Basically, McKenna argued that the singularity is actually the eschatological state in which the end of the universe is identical to its beginning. That is, he believed the height of disorder and complexity which is necessarily expressed at the end of time is simultaneously the origin of the purest order and simplicity at the beginning of time.
In a sense, all existence is in the shape of an uroboros: a snake eating its own tail. The totality of all that is, therefore, is something that exists in-itself completely formed in some exotic and utterly imperceptible non-dimensional state. In this state, one may speculate about reciprocal influences being possible across space and time. If this were true, the future could be directing the course of past events in some incomprehensible sense. With respect to ultimate reality, it could be the case that our present universe is merely one iteration in a potentially infinite sequence. We might even say that ultimate reality is conserving or perfecting itself through its prior developments. And thus, what seems impossible about the beginning could be explained by an infinite circular recurrence wherein the origin of all things is called forth by the final destination of all things.
While this is highly speculative, I don't consider this to be religion or mysticism. As I see it, this is simply an inference to the most plausible explanation given our current state of knowledge.