This is why there is still a lot of atomic hydrogen in interstellar space. Thermodynamics suggests the atoms should bind together and form molecular hydrogen,

This is why there is still a lot of atomic hydrogen in interstellar space. Thermodynamics suggests the atoms should bind together and form molecular hydrogen, but since a hydrogen molecule has no dipole moment, two hydrogen atoms alone in space cannot easily radiate away the energy released when they would bind. So they don't bind.