Why do physicists believe protons and electrons are present in equal numbers? I tended to consider that negative and positive charges are present in equal numbers in the universe to be a known, obvious fact. But is it so? How can we rule out the possibility that there is some kind of asymmetry in the numbers of protons and electrons? Of course, matter is neutral and even tiny deviations would give rise to enormous forces... But not all protons and electrons are in atoms in the universe. More importantly, is there some conceptual reason why we believe that this equality is exactly perfect or do modern theories allow tiny deviations?
 A: Well, I for one as a physicist do not believe this because conservation of charge does not mean conservation of baryon or lepton numbers. It just means conservation of charge. Physicists do not "believe" in equal numbers of protons and electrons except when bound in nuclei.
The numbers can increase arbitrarily by pair creations , so one could have any number of electron positron pairs , thus getting more electrons, and any number of proton antiproton pairs ( etc for the unstable particle)  thus getting a different number of protons even if in the beginning all charge were zero; though it will be energetically more probable to create electron positron pairs than baryon ones.
The real problem in the numbers is why the universe is predominantly of matter ( protons neutrons) and not antimatter, and this has not been resolved as yet, because the experimentally observed up to now CP violation is not enough to explain the asymmetry.
A: I think one of the reasons could be that charge cannot be created. So if initially universe was neutral (at the time of big-bang) it must remain neutral.
