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.