There are, of course, many speculations about the origin of the Universe. However, if we restrict ourselves to sweep and coherent speculations the number is drastically reduced.
There are at least two main problems: the first is speculating about the origin of the universe, or the origin of everything. In this respect (following Aristotle) either we either have a speculation with an infinite succession of causes or we speculate an initial cause. But how can we conceive an initial cause which needs no previous cause to come to be? To simply state that these set of laws (gravity and quantum physics) or this deity already existed eternally does nothing to eliminate the mystery of "why is there something instead of nothing?" For the God or set of laws might conceivably not have existed in the first place, and why it exists continues to be a mystery. This applies as well to a speculation based on the Tao as an undefinable origin of everything, as to Stephen Hawking's speculation that the laws were simply there and that they gave rise to time, so that the question "what was there before" is meaningless because time did not exist. In both cases the mystery persists: "why is there something instead of nothing?"
So, it is easy to see, that, even if we allow ourselves all the fancies of the richest imagination , it is far from easy to simply conceive of a consistent and sweep speculation regarding the origin of the everything. It is not just a matter of confronting our ideals with reality, it is not the question of the speculation being in fact true. But the simple act of imagining a consistent and sweep speculation, even putting aside all considerations regarding its truth, seems quite difficult.
In my view, the speculation that is the least unsatisfying regarding the origin of everything is from Anselm and repeated by Descartes and others. Although the idea was framed in a time full of dogmas, fear and superstitions (in an Age, in my view, correctly called "the Dark Ages") and is, correspondingly, written in a language that inspires these fears and superstitions, the idea itself is quite beautiful and it has nothing to do with the idea of the Christian or personal God, or even the Tao, etc. The idea is simply that perfection cannot not exist, or it wouldn't be perfect. Of course, if we cannot define what this "perfection" is, the speculation itself becomes so vague as to be meaningless. So, by perfection, we should perhaps say that it is anything which is not incoherent or does not self-destruct.
For instance, our best physical theories in the last hundred years or so, makes us suppose that everywhere, at all moments, there are spontaneous creations of pairs of particles and anti-particles, with almost all of these pairs simply annihilating themselves before interacting with anything else in the universe. The energy needed for their spontaneous creation was given back through their destruction. Nothing was gain, nothing was lost, but because they were possible, they have come into being.
In our current theories the uncertainty of described by quantum mechanics is responsible for this spontaneous, permanent state of creation and destruction. And the more we reduce the time frame, the higher the energy of these infinite spontaneous events of creation and destruction. Our point is that, whatever is possible will happen. And what is possible is what is not inconsistent.
So, to dissolve this mystery of "why is there something instead of nothing?", we might just invert the question: "how could there be nothing instead of something?" It seems that nothingness would require some sort of explanation too. Of course by eliminating everything we would eliminate the subject, the question, the mystery. But the inexistence of an asking subject does not reduce the mystery of the absence. Why would absence be less mysterious than the presence.
Imagine that we represent nothingness by number 0, and then we represent each possible universe by a different number. If in fact 0 was the case why would this be less mysterious than universe 23 or 3453 or some other state of affairs.
What would indeed be strange would be the existence of a universe which was inconsistent, for the existence of one part would imply the unexistence of the other. And that would be strange. But whatever is not inconsistent would need an added reason, an external reason, not to exist.
And that is perhaps the fundamental rule of the universe: simplicity - nothing is forced into being or prevented from being, whatever may happen or exist will happen or exist. This is consisten with the observation, in our universe, of such tight rules, which provides almost a complete deterministic picture of the world in which we make sense. Conversely, even if anything that is possible exists, we will only observe those things whose observation is compatible with our own existence. This means that we only see a very small part of what there is.
One of the limitations regards the physical relation between our bodies and the bodies with which we can make contact. For instance, future bodies, or bodies that fall outside the scope of our senses or instruments that expand them. Another limitation is regarding phenomental consciousness, because our own internal phenomenological experiences, are not compatible with the experience of all the possibilities.
For instance a wall may have many different colors, but a wall all painted in green cannot be seen at the same tima a In fact we cannot even think of a square and a circle at the same time. Our consciousness has a limited ability to focus. So, when we see a rock we do not see water, and we experience yellow we do not see blue. Our existence is certainly (a fortiori) a possibility of the universe, but it is a possibility that denies
and We should say just in passage that one of the most common, that it was created by Divinity, is not properly an explanation since we would have to explain how the Divinity itself started.
Aristotle has clearly stated the problem: either we have an infinite regression of causes, either we have a first uncaused cause. In either case we end up in a mystery. (See Wikipedia's Cosmological argument)
Today our notion of causality has been expanded by both general relativity and quantum physics. General relativity, for instance, allow us to think in "closed timelike curves", which are finite sequences of events that may be causing themselves. It is like a particle whose actions create its own existence.
So perhaps the Universe is, on the whole, something like that. In the end some sort of intelligence will create the conditions that will make possible for this Universe to have come into existence in the first place. Their last moments will recreate the first moments and so, will allow them, and us, to exist.
A smaller Universe would probably not be self-sustaining. We need vast numbers of galaxies and countless eons so that the basic elements of the Universe (particles) will have time to create, almost by pure chance and over the course of many eons, extremely complex structures that will be able to recreate the whole story again.
Of course, a circular causation is also no explanation. Looking at it everything seems very reasonable: we can explain every event by a certain context. We never lack events, we never lack explanations, everything is clear and self-contained.
But we are still not satisfied, for the question that rendered the Divinity answer so unsatisfying also renders this picture unsatisfying: what allowed for the Divinity to come into being? What allowed the circular set of events to come into being?
An irrational explanation is simply that there is a creative power of creating something out of nothing. We experience it in our own lives, we see it happening in quantum physics (spontaneous creation of pairs of matter and anti-matter at a quantum scale, and also non-determinism).
So although we cannot possibly understand the "creation ex nihilo", perhaps it is there. Like the number pi or the square root of minus one. These are things which are irrational, they certainly surpass the limits of reason, but nevertheless they are there and play an essential role.
On this basis we will accept as plausible the speculation that something may be created out of nothing. This of course, lends acceptance both to the circular causality hypothesis and to the First Cause hypothesis. For perhaps this first cause is something that had the ability to create itself out of nothing, just as the circular set of events created itself out of nothing.
But the most interesting thing is how this "creativity" principle is so closely related to the "coherence" principle. We can create a sound very easily, and we can even create noise very easily. But it is more difficult to create a melody that makes sense and endures through time.
The longer the story, the more complex, the more coherent it must be. And the universe we see around us, encompassing all our creations, is necessarily the most coherent, complex and encompassing thing imaginable by us (as far as we can imagine it).
This relation between creation as something coming from an irrational inspiration, and the breadth and scope of the world we see around us, speak highly of an imaginable Creator.
On the other hand, whatever our speculations may be regarding the origin of order or harmony in the universe, it is quite undeniable that it is all around us, pervading our existence at whatever scale we might want to use. In this ordered universe some stuff has a future role. For instance, light, some kinds of atoms, some sequences of molecules, some ADN, some cells, some organisms, some ways of thinking, seem to be the building blocks of these future beings, while others are simply there for a time, we might almost call them deviations, there only while the context lasts.