The period after recombination occurred and before stars and galaxies formed is known as the "dark ages". During this time, the majority of matter in the universe is neutral hydrogen. This hydrogen has yet to be observed, but there are experiments underway to detect the hydrogen line produced during this era. The hydrogen line is produced when an electron in a neutral hydrogen atom is excited to the triplet spin state, or de-excited as the electron and proton spins go to the singlet state. The energy difference between these two hyperfine states is 5.9 × × 10 − − 6 electron volts, with a wavelength of 21 centimeters. At times when neutral hydrogen is in thermodynamic equilibrium with the photons in the cosmic microwave background (CMB), the neutral hydrogen and CMB are said to be "coupled", and the hydrogen line is not observable. It is only when the two temperatures differ, i.e. are decoupled, that the hydrogen line can be observed.