As the proton and anti-neutron are not particle/anti-particle pairs there won't be a complete cancellation.

Noted Dan. I think what's interesting is that the charged pion decays into a muon and a neutrino, then the muon decays into an electron and more neutrinos. So all you've got left in your hand is an electron. Everything else has departed at the speed of light. Then if you had a positron handy, you could annihilate that electron. Then your proton and your antineutron have been rendered down to photons and neutrinos. It's as if these are the lowest common denominators of matter.

Anyway, the thing I was getting at with the positronium is that baryon asymmetry is matched by lepton asymmetry, and that there’s an ambiguity when it comes to matter and antimatter. Let's forget about neutrons for a minute, and focus on electrons and protons and their antiparticles. If we have four particles

*a b c d* we could label two of them matter and two of them antimatter. Let’s say

*a* and

*b* are matter and

*c* and

*d* are antimatter. We can then pair up those four particles in six different various ways:

*ab ac ad bc bd *and

* cd*. If we could somehow stick the two matter particles

*a* and

*b* together we could reasonably call the result “matter”. There’s only one way out of six that we could do this:

*ab*. If we could somehow stick the two antimatter particles together we could reasonably call the result “antimatter”. There’s only one way out of six that we can do this:

*cd*. However the other four combinations

*ac ad bc bd *consist of both matter and antimatter. So there’s twice as many ways to make an exotic atom as there are to make an atom of matter or antimatter. The mystery of the missing antimatter is that there’s more particles than antiparticles in the universe. But if we took a tip from positronium and said hydrogen was an exotic atom too, we’d have the

*same* number of particles and antiparticles in our universe. Like this: