Immediately after the Big Bang, which occurred around 13.8 billion years ago, the universe was dominated by unimaginably high temperatures and densities. However, after just a few seconds, it had cooled down enough for the first elements to form, primarily hydrogen and helium. These were still completely ionized at this point, as it took almost 380,000 years for the temperature in the universe to drop enough for neutral atoms to form through recombination with free electrons. This paved the way for the first chemical reactions.
The oldest molecule in existence is the helium hydride ion (HeH⁺), formed from a neutral helium atom and an ionized hydrogen nucleus. This marks the beginning of a chain reaction that leads to the formation of molecular hydrogen (H₂), which is by far the most common molecule in the universe.
Recombination was followed by the “dark age” of cosmology: although the universe was now transparent due to the binding of free electrons, there were still no light-emitting objects, such as stars. Several hundred million years passed before the first stars formed.