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Protons and neutrons form nuclei

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. At this point in the expansion and cooling of the Universe, the average temperature is low enough so that neutrons and protons can stick together and make nuclei of the lighter elements such as hydrogen, helium and lithium. Physicists call this process nucleosynthesis, and it had to occur before the structures we observed today, such as atoms and molecules, could exist.
. Neutrons and protons only attract each other at very short distances, less than 10-13 centimeters. The strong nuclear force that holds them together is confined and cancels out at larger distances. So in order to form nuclei, neutrons and protons have to spend some time in very close proximity to one another. This can't happen if the temperature is too high, because then the protons and neutrons will be moving too fast to spend much time near one another.
. The majority of the neutrons in the Universe wind up stuck in combinations of two protons, two neutrons, in the helium nucleus. A few neutrons contribute to lithium, with three protons and three neutrons, and the leftovers wind up in deuterium, which is an isotope of hydrogen with one proton and one neutron.
. Nucleosynthesis sets the stage for the formation of atoms and then galaxies and stars.

How old is the Universe? // Structure of the Universe // The Big Bang // Before the Big Bang? // What about string theory?

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