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One hundred seconds after the Big Bang, the temperature dropped to the point where protons and neutrons could stick together without being torn apart by highly energetic photons. The neutrinos and antineutrinos had lost their ability to interact with protons or neutrons, and all of the positrons (electrons' antimatter particle), had been annihilated.
The conditions were suddenly ripe for nucleus formation, the most stable being that of two protons and two neutrons (helium). At the end of the nucleosynthesis period, all of the neutrons had paired with protons to form helium (24% of the primordial light elements) and trace amounts of deuterium (2 protons), tritium (3 protons), helium-3 (two protons and one neutron) and lithium (three protons and four neutrons).
The protons that were left over were destined to become hydrogen nuclei, which made up 75% of these new atomic nuclei. These nuclei, being composed of baryons, are known as baryonic matter. The ultimate fate of the universe--whether it continues to expand forever, eventually reaches a steady state, or collapses in one big crunch--depends on the density of baryonic matter.
Astrophysicists at Johns Hopkins University recently detected, in the intergalactic medium, the helium formed in the first two minutes after the Big Bang. This matter, along with the primordial hydrogen that is almost sure to accompany the helium, is sparsely scattered throughout intergalactic space.
These atomic nuclei--joined with electrons many years later--would eventually become the seeds of stars. All of the other elements--from the carbon, nitrogen, and oxygen upon which life is based to metals like iron, copper and gold--were forged in repeating cycles of starbirth and death. And, although stars continue to produce helium, scientists believe that 98% of the helium in the universe today was produced in those first few seconds.
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