We're pretty sure they're (most likely fully ionized) atomic nuclei, but we still aren't sure which ones.

Resolves to PROB based to the best estimate available (marginalized over uncertainties) on the market closing date of the number fraction (i.e. one hydrogen-1 nucleus and one helium-4 nucleus count as 50% H + 50% He, not 20% H + 80% He).

For the purposes of this question, a free neutron would count as a proton (hydrogen nucleus), an antimatter nucleus would count as its antiparticle, anything that does not contain an atomic nucleus (e.g. a gamma ray or a neutrino) would not count as a cosmic ray, and anything that contains several would as several cosmic rays (e.g. a 200 EeV HF molecule counts as a 10 EeV hydrogen nucleus plus a 190 EeV fluorine nucleus, of which only the latter passes the 100 EeV threshold).