The recombination epoch begins at around 18,000 years, as electrons are combining with helium nuclei to form He. At around 47,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. Much later, hydrogen and helium hydride react to form molecular hydrogen (H 2) the fuel needed for the first stars. At about 370,000 years, neutral hydrogen atoms finish forming ("recombination"), and as a result the universe also became transparent for the first time. The newly formed atoms—mainly hydrogen and helium with traces of lithium —quickly reach their lowest energy state (ground state) by releasing photons (" photon decoupling "), and these photons can still be detected today as the cosmic microwave background (CMB). This is the oldest direct observation we currently have of the universe.