The most abundant element in the universe is Hydrogen, the second most abundant element is Helium. A great success of the Big Bang theory is to be able to predict the relative amounts of these elements: after the universe cooled down sufficiently protons and neutrons were able, after a collision, to remain in the form of heavier atomic nuclei, in this manner Helium and Lithium were created, and also Deuterium (whose nucleus has one proton and one neutron). The universe was 1s old, its temperature was 1010 oK.
It was initially thought that all elements would be generated by the Big Bang, but this is not the case: even at the extreme temperatures available when Helium and Lithium nuclei were crated, this was not enough to smash two Helium nuclei to create something heavier, the creation of the remaining elements of the periodic table had to await the appearance of the first stars (see Sect. 9.3). Deuterium and Lithium, while used up in stars through the nuclear reactions that make them shine (see Sect. 9.2), are very rarely created by them. Whatever Deuterium and Lithium we see in nature was created about 15 billion years ago. Most of the Helium we observe (even though it is manufactured in stars) also came from that epoch.
The Big Bang theory predicts is the relative amounts of Helium and Lithium and Deuterium and Hydrogen. And the observations match the predictions; for example there are about 4 atoms of Hydrogen for each one of Helium. These same calcualations predict that there are 3 light neutrinos, again confirmed by observation.
Coupled with our understanding of stellar processes and evolution (Sects. 9.2 and 9.3) we now understand the manner in which all elements in the periodic table were created. This is one of the most important predictions of modern cosmology.