The first atomic nuclei to exist in the very earliest moments of the universe were

In the beginning of the universe, the universe is unstable mainly because of the extreme energy and heat in a small amount of space. As the universe expanded and cooled down, subatomic particles such as quarks which led to the formation of bigger subatomic particles such as the electrons and protons. There came a time when the universe has cooled enough that electrons and protons can form atoms, and the first atomic nuclei formed by 1 electron and 1 proton is the hydrogen atom. 

The next atomic nuclei formed is helium which is formed by 2 electrons, 2 protons, and 2 neutrons. 

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