The cosmic microwave background does not originate with the big bang itself. It originates roughly 380,000 years after the big bang, when the temperature dropped far enough to allow electrons and protons to form atoms. When it was released, the cosmic microwave background wasn't microwave at all- the photons had higher energies. Since that time, they have been redshifted due to the expansion of the universe, and are presently in the microwave band.
The universe is opaque from 380,000 years and earlier. The galaxies that we can see only formed after that time. Before that, all that is observable is the CMB.
The Planck era is defined as the time when the universe was the size of the Planck length, $10^{-33}$ cms, and less, and the universe's age was $10^{-43}$ sec, the Planck time, and less. It is the earliest epoch we identify after the Big Bang. The Planck temperature at the end of the epoch was about $10^{32}$ degrees Kelvin.
This was way before quarks, leptons, Higgs bosons, and inflation. Neither Quantum Theory (QT) nor General Relativity (GR) have anything to say about what is happening at these sizes, times, energy densities and temperatures, except that they are not applicable.
The wiki article summarizes the epochs, or times, when the universe was dominated by different kinds of physics, from the Planck epoch through the GUT epoch, inflation, electroweak and strong force separation, and on till the current epoch. It is at https://en.m.wikipedia.org/wiki/Chronology_of_the_universe
At Planck times and earlier after the Big Bang, GR effects or equivalently strong gravity effects predicted by GR, would be happening at the sizes and energies such that QT also applies. It needs a unification of GR and QT to have anY explanatory power, a theory of quantum gravity. At these times there are no photons, quarks, electrons, neutrinos, gluons or anything associated specifically with the 4 forces. This was before spacetime was defined by geometry and GR, or even before it existed. The two best known theories of quantum gravity, none proven or accepted, are string theory (ST) and its newer versions of superstrings and M theory, and loop quantum gravity. They both have elementary entities which can be about the Planck size, one is strings and the other loops.
I know quantum loop gravity less well, but when the spacetime is larger than the Planck length ST has strings in a 10 or 11 dimensional spacetime, which were all small dimensions unti the inflationary era where for some reason all but 4 dimensions remained small, and so now we see those 4 dimensions. At least that is one version. Others take multidimensional branes in a higher dimensional space. You can see the wiki references on string theory. But either way at the Planck length it becomes a sort of quantum foam, with no spacetime defined. Spacetime emerges as the scale factor increases and multiple Planck lengths enter in.
So, what happens at the Planck epoch (era) is not well understood. As the universe goes into the next era, the GUT era, 3 of the 4 forces, all but gravity which when we enter the GUT epoch decouples, are unified. ST aims to also explain it, i.e., a unification of the 3 forces (and actually gravity in the Planck era as well). The 3 forces then remain unified until the universe expands and cools some more, the strong force decouples, and later there is inflation and the electroweak force separates into the weak and electromagnetic force, and the Higgs boson emerges. There is more after that.
See the wiki and its references for string theory cosmology, but at inflation and later (maybe even before, up to the Planck epoch) there is commonality with the standard cosmology model.
https://en.m.wikipedia.org/wiki/String_cosmology
Best Answer
This seems to be a common misconception about the big bang.
At present our theories can only suggest what happened AFTER the "bang". We cannot formulate what occurred AT the singularity with our current knowledge of physics.
At a small neighborhood around a spacetime singularity quantum gravity becomes important and we simply have no clue at present how to deal with the general relativistic singularities (e.g. black holes as well!)
When you hear of physicists talking about the big bang it is almost always a discussion of the dynamics AFTER the singularity.
When physicists discuss what happened during the planck era or at a singularity it is entirely (very) educated guesswork within the bounds of current theory (much like how theoretical physicists like hawking have come up with convincing theories about some black hole properties).
Hope that helps!