system
Given the following post, respond as if you're commenting on Reddit:
human
If a black hole created from matter, and a black hole created from antimatter collide, is the result a bigger black hole or would something else happen?
gpt
A black hole built from antimatter or one built from matter with the same mass are identical.

The quantity that distinguishes matter from antimatter is the baryon number. It is the total number of baryons (like protons and neutrons) minus the number of antibaryons. Baryon number is conserved in the standard model and that is what you would recognize as "conservation of matter" in some sense, and the reason for which the proton is stable in the SM: it is the lightest baryon and so it cannot decay to anything else because it needs to preserve its baryon number of +1. So the proton can only release its mass in energy for lighter particles if it meets an antibaryon carrying -1 baryon number, so they total to 0 and they can produce lighter non-baryon particles such as photons or mesons. That's why matter & antimatter annihilate.

(A similar argument works with leptons, with a thing called lepton number).

These, baryon and lepton #, however, are not conservation laws with any specific deep or fundamental justification behind, they are just accidental symmetries of the standard model. (Unlike, for example, charge). As soon as you step out of the SM, these are violated. Black holes are well outside the SM and do not care in the slightest about baryon number. Black holes don't have a well-defined baryon number and throwing a proton or an anti-proton\* into a black hole produces the same final black hole, whose only macroscopic distinguishing properties are mass, charge and angular momentum. After a lot of time the black hole has evaporated into Hawking radiation which is made of standard model particles, and the final baryon number of the radiation does not have any discernible relation with the baryon number of all the stuff that fell in. 


\* woops forgot: except for charge, charge is conserved even by BHs. So maybe you can throw a hydrogen atom (baryon # 1, charge 0) or an anti-hydrogen atom (baryon # -1, charge 0). Or neutron/antineutron.