Which brings us to today.

The universe then entered a period of nuclear interactions, the radiation era. This era lasted from 10 seconds to 10⁶ (1 million) years. Neutrons and protons reacted to form the nuclei of simple elements, including deuterium, helium, and tritium.

After approximately 1 million years, the temperature fell to about 3000 K, and nuclei began to capture electrons, forming the first atoms. Almost all of these atoms were either hydrogen or helium. This marked the beginning of the matter era. After a thousand years or so, matter became transparent to radiation, and light broke through the now transparent matter. Matter, freed from its coupling with radiation, clumped together to form the first stars and galaxies, and the radiation left over cooled to 2.7 K. The Milky Way formed at about 5 billion years, and our solar system condensed at around 10 billion years.