The lack of a good answer to these questions implies the presence of God, at least as the first cause. God (in the religious sense) is most easily defined as the creator of those forces which are beyond our comprehension. I have no problem with that, as long as it stops before it becomes God the continuing manipulator. Such concepts tend to conflict with my worship of the laws of cause and effect. The concept of God seems to be shrinking steadily, but the creation of the big bang will be its last big stand. I find it acceptable to say that perhaps God, consciously, intentionally, or otherwise, transformed "his" possibly infinite energy into the expanding universe, or that he started the experiment running, and with Godly wisdom, chose to stay out of it. It makes little difference which way you go. A loving God wouldn't "keep his hand in", changing the rules every now and then in favor of those who worship him. This leads to trauma and insanity. If God still has final control, my sentience loses its meaning.
My conventional understanding of the big bang is that it started as an infinitely dense body of infinitesimal size, and probably began expanding at the speed of light. Starting out as just a point, it was necessarily uniform. From the theories/guesses I've heard, it stayed uniform for a few seconds after the beginning. Why then did it become non-uniform?
I don't hope to give a complete answer, but I have a hypothesis about the macro-cosmos part. Why are there galactic clusters and clusters of clusters and voids between them? Perhaps the following ideas also give some insight about other scales of the universal non-uniformity. I'm not an expert in quantum physics so I don't know how much my speculations agree with others' views.
Quantum theory seems to explain, at least to the satisfaction of quantum physicists, why subatomic particles would condense from seemingly uniform energy. They say that energy is basically granular and only occurs as particles. Though light seems to spread uniformly from a source, when you go looking for small amounts of light, you find it in little bundles, called photons. The same rules apply to all other kinds of energies. But at first it seems this should only apply to sub-microscopic particles and not be noticeable in the formation of stars and galaxies.
In "A Brief History of Time", Steven Hawking implies that discontinuities and random positions and velocities of sub-atomic particles added together to produce the same on macro scales. To me, this clearly goes against the Second Law of Thermodynamics. A star or galaxy is clearly a concentration of matter/energy. Therefore its formation seems to go against entropy. On the other hand, even as I write this, I realize that totally even dispersion of matter is a very unstable state, possibly upset by even sub-atomic randomness.
But my recent "AHA" on this, which I think I gained on my own while reading the above book, is as follows:
At some point, one generation of particles divided into what later became matter and anti-matter, which went their separate ways.
I won't try to guess how many generations of particles went by, but eventually each particle had the mass, though not the form, of what we now call a cluster of galactic clusters. They continued to split into single particles with (only) the mass-equivalents of clusters, then galaxies, then stars with planetary systems. Eventually we get to the stage of quarks (still somewhat hypothetical). But each of these different sized clusters of matter/energy was represented by one particle at some point in the universe's evolution. As these separated, they formed discontinuities and seeming randomness on a large scale. Thus stars, galaxies, galactic clusters etc. represent, quantum particles that existed when the universe was very young. This means they couldn't have filled space uniformly.
Quarks then condensed (or split?) into the particles scientists "see" (or in some cases, hypothesize) today, such as photons, neutrinos, neutrons, electrons, protons. The latter further condensed into hydrogen atoms. The mass concentrations caused by the above evolution of splitting particles caused the hydrogen to condense to stars, galaxies etc. Some of the stars exploded as supernovas, condensing hydrogen atoms into heavier elements. These, and more interstellar hydrogen, condensed to form "second generation" stars and planets, which is where we are today.
Is this any more nosensical than conventional cosmology or quantum physics? Keep in mind that scientists don't generally talk much about the size of quantum particles, and their mass isn't necessearily related to that of particles they condensed or split from.
Another view could be that particles didn't really split, but rather, as time progressed, the rules pertaining to different levels of particles were emphasized. In either case, perhaps there was no beginning and there are infinite generations yet to come.