Why is it assumed that the Big Bang produced equal parts matter and antimatter? It is well-known that a major open question in physics is why the Universe appears to be made almost entirely out of matter, with next to no antimatter, despite the two being strictly symmetrical under the standard model. I know that it has to do with the breaking of the CP symmetry in some way. When researching it, I keep being told in the first few sentences something along the lines of "at first, there was as much matter as antimatter". But why? What is never explained in my course material or any popular treatment of the issue is why it is assumed that the Big Bang would result in a 50-50 ratio in the first place, which would then require some other as-of-yet-unknown mechanism to create an excess of regular matter and break the symmetry.
This is what I don't understand : since we are discussing the boundary conditions of the Universe, why couldn't the initial ratio be set arbitrarily? In fact, we already observe a lack of antimatter, so wouldn't a Universe that was always mostly matter from the start be the more parsimonious assumption? In short, which part of the theoretical framework of the standard model makes us expect the early Universe to have been symmetrical in contents, rather than simply containing excess matter "because it is so"?
(As an aside, yes, "because it is so" is a frustrating answer to any scientific inquiry, but it has to come down to it eventually. Conceptually, a perfectly balanced Universe is attractive because of the simplicity and elegance of the math behind it, but we already know from the very fact we exist that it couldn't be perfectly symmetrical, or it would be empty and unchanging. What difference does it make if we put that asymmetry in the boundary conditions instead of the physical laws?)
Besides, in a naive many-worlds thought experiment, it seems to me that, the Universe being of gigantic but unknown size and the original amount of matter prior to anihilation also unknown, almost any initial ratio barring a nearly-perfect balance would have resulted in a mostly-matter Universe (if antimatter had won out, we would just have an inverted terminology), with the abysmally minor alternative being an empty Universe. So this does not even feel like a case of fine tuning. The outcome of perfect symmetry would have been much more "unlikely". I assume there is a flaw in that line of reasoning, of course, but it is unclear to me. Could someone with a deeper understanding of the field point it out, please?
I understand that this is a very ambitious question, so please forgive me if it is advanced beyond my capabilities. To give you a sense of my level, I am about equivalent to a Master's degree in physics, so I do understand the basics of the standard model and some of the principles of higher-energy unification, but I did not study any of the alternative theories.
 A: The strongest motivation for initially equal amounts of matter and anti-matter actually comes not from the standard model but from cosmology - there is a staggeringly large number of photons in the Universe, relative to baryons$^1$. It's possible to arrive at this conclusion a couple of different ways. One is to look at the cosmic microwave background, which is the single largest contributor to the overall photon number density in the Universe. It has an energy density of about $0.25\,{\rm eV}\,{\rm cm}^{-3}$, which works out to about $n_{\gamma}=500\,{\rm photon}\,{\rm cm}^{-3}$. Compare this to the baryon energy density of about $240\,{\rm eV}\,{\rm cm}^{-3}$, which works out to $n_{\rm bar}=2.6\times 10^{-7}\,{\rm proton}\,{\rm cm}^{-3}$. There are therefore about $n_\gamma/n_{\rm bar}\sim2\times 10^9\,{\rm photon}\,{\rm baryon}^{-1}$.
The argument is then that at early times there were near-equal amounts of matter and anti-matter, most of which (except for about one part per billion) annihilated into photons. This either leads to a fine tuning problem - why was there a $10^{-9}$ excess of matter over anti-matter in the initial conditions? - or you can instead try invoking a CP-asymmetric process to break the initial matter/anti-matter symmetry. Most physicists and cosmologists feel more comfortable with the latter option; we tend to be very fine-tuning averse.

$^1$I'm using the astronomer's baryon here, which is a loose term encompassing more or less all non-relativistic matter (mainly atoms, leptons), but not (cosmological) dark matter, neutrinos, or photons.
A: The mathematical meaning of a singularity is that we cannot define mathematical quantities. Since we cannot define mathematical quantities, we have no idea of the physics in the initial singularity. Anything we assume is therefore a speculation. We cannot know whether the universe was created out of nothing, and, if so, whether it was created with a matter-antimatter imbalance, or whether there was something before the initial singularity which gave rise to the matter-antimatter imbalance.
We do know, because of the very high energies and densities just after the initial singularity, that matter and antimatter existed in near equal quantities, and we do know that after it annihilated there was a small proportion of matter remaining. We do not know how this happened.
We can, however, make assumptions, and attempt to deduce what the conclusion of those assumptions would be. Such assumptions are not scientific theory, they are hypotheses which we can attempt to test.
One assumption is that matter and antimatter should have been created out of nothing in precisely equal quantities. There are a number of problems with this assumption, but it has the merit of fitting with what we know of pair creation. On the other hand it violates any form of conservation of energy which we can reasonably formulate, and it necessitates postulating some unobserved physical process to explain the matter-antimatter imbalance - although, at least superficially, such a process would violate the general principle of relativity that laws of physics are everywhere the same.
Personally I don't like that assumption. I prefer to think that the Big Bang followed from some earlier, unknown, state (perhaps a cyclic universe, although that necessitates explanation of why observed cosmological parameters appear not to fit a cyclic universe).
Whichever assumption is chosen, one should be aware that it is only assumption, speculation unsupported by evidence. Consequently one should be prepared to abandon it if evidence is discovered to the contrary. All I would say, is that if an author presents a speculative assumption as though it is established science, you should immediately mistrust that author's scientific judgement.
