Could the universe's antimatter be hiding in black holes? As far as we know almost all the mass in the universe is matter, not antimatter. There are three scenarios:


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*The universe started off with more matter than antimatter and the small amount of antimatter has mostly been converted to energy via collisions with matter

*The universe started off with equal amounts of matter and antimatter and some unknown process caused the situation we see today

*The universe started off with less matter than antimatter and somehow the ratio reversed (I've never heard this discussed as it seems so unlikely)


We reason that mass is matter because if there were large amounts of antimatter they should produce antimatter galaxies, which would give off huge amounts of energy when they interact with matter galaxies. This has never been observed so we conclude that all galaxies, therefore all mass in the universe is matter.
As I understand it there are two main types of black holes, stellar remnants (the collapsed remnants of dead stars with more than two solar masses) and super-massive black holes at the centre of many galaxies, including one thought to be at the centre of our own galaxy.
Stellar black holes are obviously originally composed of matter, for the same reason that all galaxies are composed of matter.
We don't know how super-massive black holes formed. Is there enough mass in them to contain all the antimatter in the universe if matter and antimatter were originally created in equal quantities? I realise this shifts the unknown mechanism from "why was more matter created than antimatter" to "why is antimatter more likely to form super-massive black holes" but is this hypothetical scenario possible?
 A: If we take the Milky Way as an example, the black hole at the centre, Sagittarius A$^*$, has a mass of about 4 million times the Sun. However the mass of the Milky Way is somewhere around a trillion Suns. So the central black hole makes up 0.0004% of the total mass. So even if our central black hole was pure anti-matter it wouldn't come close to accounting for all the anti-matter as you suggest. Of course this is just our galaxy, but there's no reason to suppose other galaxies are qualitatively different.
For completeness I should note that visible matter makes up less than 10% of all matter, with dark matter making up the balance. This makes supermassive black holes an even tinier proportion of the all (anti)matter.
A: The black hole has lower mass than the overall galaxy. So it is not believed that black holes have the antimatter. However the process by which more matter happened to exist was because of a process called leptogenesis, and baryogenesis which produced more matter than antimatter. Maybe supermassive black holes could have absorbed some amount of antimatter but that is not the main way matter won out against antimatter. Also most black holes formed after the collapse of a star(stellar black holes) or a dust cloud(supermassive black holes). Note there are probably otherways supermassive black holes can form. These things were mostly matter. So even if the black holes were a huge proportion of the galaxy's mass(which it is not, but lets say) then it is still not going to acount for the matter antimatter assymetry because the assymetry was formed a long time ago.
https://en.wikipedia.org/wiki/Leptogenesis_(physics)
A: In a dual time universe, black holes are the antimatter induction terminus of the graviton cycle that powers all atomic and galactic motion.  So, what we have is a reverse motion antimatter core that strips gravitons from a forward motion wall and in turn the graviton's flip polarity and become antigraviton's that are being broadcasted at frequencies above our light spectrum back towards our reverse time antimatter partner.  We can see the FTL shock waves in the 50,000 light year accross FERMI BUBBLES that are located above and below the SAG A galactic core which is one half the distance of our 100,000 light year across Milky Way bar galaxy.  It is the repulsive force of the antigraviton's leaving cores of galaxies and cores of Proton's that create the force of gravity.  Mass is a by-product of this graviton cycle, and mass has nothing to do with the creation of gravity.  
We are extremely naive about the functions of black holes.  There is no such thing as a singularity, all we are seeing is a large scale energy exchange taking place causing mass to form in arms of galaxies.  This forward time motion appears to be in an accelerated expansion, and the CMB is cooling down. We are unpacking from around a core, and the super arm is flattening out and most likely the 13.75 billion year evidence that galaxies are accelerated at faster then light away from each other is a lensing/curvature problem from our traveling around a torus.  Also, CMB slices show parallel lines that never meet, indications that we are moving around a toroid. However, the primary point here is that MASS could not exist at all if there was not a continuous power source to cause mass to exist.
I doubt that there is anyone on the planet that even comes close to understanding what I just said.  But, it is the truth, and we have a long ways to go in truly grasping how our Universe works.
