The “cooling limit” of this species corresponds to a present-day halo mass of around 10 to the 8 solar masses, problematizing scenarios of “monolithic” or “direct” collapse. In 1969, Field, Goldsmith, and Habing computed the cooling curve of this species to show that its cold and warm phases coexist in a two-phase model. The excitation temperature of this species can exceed the CMB temperature due to Wouthuysen (“VOWT-high-sen”) –Field coupling. At redshifts (15[1])greater than (*) 6, absorption by (-5[1])this species creates a Gunn-Peterson trough (10[1]-5[2])since (10[1])this (-5[1])species was ubiquitous during the Dark (-5[1])Ages. This most abundant species in the ISM is typically observed via a hyperfine transition (-5[1])of its ground state, (-5[2])in which a spin-flip causes emission of a 21 centimeter (-5[1])photon. For 10 points, name this species containing one proton and one electron. ■END■ (10[2]0[7])