What is the most common form of antimatter in the universe? During our high energy accelerator collisions we are bale to produce antimatter. Processes in the Sun produce positrons. Neutron decay produces anti-neutrinos. Cosmic rays can produce a myriad of particles. What is the most abundant antiparticle in the observable universe? The Photon not withstanding.
 A: Assuming the observed asymmetry in matter and antimater in the universe the antimatter to be detected must come from the interactions of matter particles with excess energy, creating a particle antiparticle pair.
The lightest pairs are  a pair of neutrino antineutrino. To be created, the weak interaction vertex has to be involved which is much smaller than the electromagnetic one involved in the creation of electron positron pairs. X rays and gamma rays have a much higher probability of creating electron positron pairs than neutrino antineutrino ones from direct creation.
It is true that neutron decays will give electron antineutrinos but still the weak vertex is involved and free neutrons are a by product of other interactions. As an estimate I would say it is the positrons that will dominate. 
A: 
What is the most common form of antimatter in the universe?

The proton. Yes, the proton. 
See this article about positronium which says "to a first approximation it can be regarded as a sort of light hydrogen atom". Positronium is a short-lived exotic atom, comprised of an electron and a positron. We call the electron matter, and we call the positron antimatter. We also call the proton matter, but actually, this is just a convention. The proton is more like a positron than it's like an electron, and positronium is like light hydrogen. So you could justifiably say the proton is the antimatter and the antiproton is the matter. Especially since baryon asymmetry is counterbalanced by lepton asymmetry. IMHO it's something like a game of mixed doubles in tennis. One side was always going to win, and did win. But then we called the winning team the matter. And now we wonder why the ladies lost, when actually, they didn't. 
On that basis the proton is the most common form of antimatter in the universe. Where did all the antimatter go? Nowhere, because weight for weight, 99.95% of you is made from it.    
