The radioactive decay of neutrons reduces the ratio of neutrons to protons to 1:7. Nearly all surviving neutrons end up in helium-4 nuclei, leaving only a few traces of deuterium, helium-3, and lithium. The universe now has 23% helium-4 matter, 77% hydrogen, and tiny traces of deuterium, helium-3, and lithium-7. This is essentially the composition still detected today in every direction.
