Supposedly, a small imbalance between matter and anti-matter gave us the universe?

Ah, one of the greatest unsolved mysteries in all of physics! If not science as a whole. Matter and Antimatter annihilate each other when they come into contact. If there had been an equal amount of each in the Universe, nothing would exist because everything would have been annihilated. Why, then, does the Universe appear to be Matter-dominated? Either the Universe began with more regular Matter, or some mechanism occurred after the Big Bang that resulted in an asymmetry between Matter and Antimatter.

I don't know of any specific theories as to why this "baryogenesis" exists. There are the three Sakharov conditions, which are supposedly necessary in order for the rate of synthesis of regular Matter be greater than that of Antimatter, resulting in more Matter in the Universe. It involves some Quantum Mechanics that I don't fully understand, but you could google "Sakharov conditions" and see if anything comes up.

Why would there have to be something to cause an imbalance? It is MUCH more likely for something random or unplanned to have an imbalance. Since they annihilate each other and we have matter, there had to be more matter at the beginning. There could have just as easily been more antimatter and we wouldn't be here right now. Grab a handful of black sand (antimatter) and a handful of white sand (matter). I think the odds of an imbalance would be MUCH greater than the odds you grabbed the exact same amount to cancel each other out.

The best guess about the imbalance is that some massive particle preferentially decays into matter instead of anti-matter. This isn't as silly as it looks at first because we *know* that there is an asymmetry between matter and anti-matter in some weak force decays (Kaons, for example, act differently than anti-kaons). The current estimates are that a difference of one part in a billion was enough to have the whole universe left over.

Your question is fundamentally off-base.

You're stating ONE theory of the cause for the big bang, not THE theory, and certainly not the most widely accepted theory.

Currently most physicists are working towards the idea that matter simply coalesced together at a single point. As more matter came together, its gravitational pull increased drawing in more matter, more quickly ad infinitum until there was a singularity where the matter that composes most of the universe as we know it was gathered into an immeasurably small space similar to what we might think of as a black hole and that the heat and pressure eventually caused it to explode outward in one cataclysmic expulsion.

The matter/anti-matter theory is a bit more "thinking out of the box" and will likely be relegated to the "Oh, well. It was a fun idea for a minute there" pile.