system
Given the following post, respond as if you're commenting on Reddit:
human
Where does the neutron in the nucleus of 
deuterium come from, knowing that deuterium is 
produced by the fusion of two hydrogen atoms 
which only contain a proton?
gpt
The weak nuclear force can turn protons into neutrons and neutrons into protons. Normally the process of turning neutrons into protons dominate completely since it requires adding energy to turn protons into neutrons. But in the early universe the typical energies of protons and neutrons was much higher and both interactions were balanced. Some of the neutrons reacted with protons to form deuterium. When the universe expanded and became cooler all the free neutrons decayed to protons. But the neutrons inside deuterium stuck around.

If you look closely at the details there are a few other ways to produce deuterium, but most of it come from the early days of the universe.