Dark Ages Radio Explorer
The period after recombination occurred but before stars and galaxies was created is called the "dark ages". During this time, most of the 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 a state where the electron and proton have aligned spins, or de-excited as the electron and proton spins go from being aligned to anti-aligned. The energy difference between these two hyperfine states is  electron volts, with a wavelength of 21 centimeters. When neutral hydrogen is in thermodynamic equilibrium with the photons in the cosmic microwave background (CMB), the neutral hydrogen and CMB are called to be "coupled", and the hydrogen line is not observable. It is only when the two temperatures are not same, or decoupled, that the hydrogen line can be observed.