Answer
The intergalactic medium is cold compared to the light-emitting galactic material.
Work Step by Step
In the intergalactic medium, most of the hydrogen atoms must be in the ground state, n=1, since they strongly absorb Lyman-$\alpha$ photons and move to the n=2 state.
The Maxwell-Boltzmann distribution (section 18.5) tells us that the atoms must be at a relatively low temperature for so many of them to be in the ground state.
