After about one to three minutes had passed since the creation of the universe, protons and neutrons began to react with each other to form deuterium, an isotope of hydrogen Deuterium, or heavy hydrogen, soon collected another neutron to form tritium. Following this was the addition of another proton which produced a nucleus. Scientists believe that there was one helium nucleus for every ten protons within the first three minutes of the universe. After further cooling, these protons would be able to capture an electron to create hydrogen. Consequently, the universe today is observed to contain one helium atom for every ten or eleven atoms of hydrogen.

Second piece of evidence