The ratios of elements found in the oldest gas clouds in the universe contain one of the primary pieces of evidence for the Big Bang. While stars turn light elements into heavy elements (and supernovae generate even heavier elements), they aren't the sole source of atoms heavier than hydrogen. This is observed most strongly in the abundance of helium. About 25% of the mass of gas in the universe is helium, but stellar fusion in main sequence stars like the Sun haven't had enough time since the Big Bang to generate this much Helium. Helium also shows a uniform distribution throughout the Milky Way and in other galaxies. By contrast, the abundance of heavier elements, which are generated in stars and supernovae, decreases with distance from the center of the Milky Way and is correlated with the number of supernovae. In other words, there is too much helium in the universe to be explained by stellar fusion — and the distribution also can't be explained with stellar nucleosynthesis. What stars and their stellar remnants can't explain, the Big Bang explains instead.
Nuclear reactions in primordial nucleosynthesis. Click here for original source URL.
Russian physicist George Gamow began speculating about the consequences of the Big Bang model in the 1940s. He realized that the density and temperature predicted for the early phases of the Big Bang would provide just the right conditions to produce helium nuclei by the fusion process. The creation of deuterium and helium, along with trace amounts of lithium and beryllium, in the first 225 seconds after the Big Bang is called cosmic nucleosynthesis.
In the first moments of the universe, the temperature was billions of degrees. This was so hot that atoms were entirely ionized. Electrons and nuclei couldn't bind together, so the universe was a dense hot broth of radiation and particles undergoing constant collisions. The entire universe was contained in a volume about the size of the Sun! After about a minute, when the temperature had fallen to a billion degrees, nuclear reactions began to take place. Initially, neutrons and protons combined to form deuterium nuclei, symbolized as 2H and sometimes called heavy hydrogen. Deuterium would then capture another neutron to make tritium (3H, one proton and two neutrons) or another proton to make helium-3 (3He, two protons and one neutron). After one more stage, helium-4 nuclei (4He) were created, using up almost all the available neutrons. This process was complete in less than four minutes after the big bang. In this brief period, 25 percent of the regular mass of the universe had turned into helium. During the next half-hour, tiny amounts of lithium-7 (7Li) and beryllium-7 (7Be) were created. After this, the reactions stopped. The universe became too cool and diffuse to synthesize heavier elements. In the simple big bang model, all heavier elements were produced much later, in the interiors of stars.
Astronomers have compared the abundance predicted by the Big Bang theory with observations for four light elements — helium-4, deuterium, helium-3, and lithium-7. Since some of these elements are created (and destroyed) in stars, pristine parts of the universe must be found to measure their abundance as left by the big bang. The abundance relative to hydrogen are 0.15 for helium-4, 2 × 10-5 for deuterium, 10-5 for helium-3, and 3 × 10-10 for lithium-7. Remarkably, all four measured abundance agree with a simple hot big bang model with only one variable: the density of ordinary matter relative to photons. Observations of the real universe match a very narrow set of theories, where there are a billion photons for every matter particle in the universe. This extraordinary success shows not only that the universe did undergo the big bang, but it shows that we can understand the details of the earliest, unobservable moments of the universe.