Matter Antimatter imbalance is a big mystery and there are some thoughts given out by many people
1) may be at the time of big-bang itself "laws of physics" were unified. And thus they may or may not be equal. and making anti-matter near to negligible in compared to the mass of matter.
2) The observable universe is basically the further area of the universe from where the light has reached us. Light may be a matter of property and what if anti-matter properties are opposite of matter's and they were the first ones to chucked out on the big bang and are further away in depth of non-observable space.
3) if it is all about balance, could there be two universes, one that of matter and one parallel but made up of anti-matter. would the time be running backward in that universe?
4) The last one on my mind is the black hole. We still have to understand them. We know they can be both large and small. We know whether small or large the mass compressed in that are is super dense. So are all blackholes actually antimatter in condensed form? and would their weight balance out the visible universe's matter mass?
So all of these above actually are in reply to the question, why there is an imbalance in the observable universe.
Instead of questioning the validity, I find questioning the "Visibility" aspect of it. It needs to be condensed, beyond the visual spectrum.