Molecular hydrogen is indeed hard to detect spectroscopically. This is what led Marmet to beleive that there is sufficient intergalactic molecular hydrogen to cause the red shift. Photons from background galaxies hit the intergalactic gas, lose momentum and energy to it to cause the red shift according to Marmet. Marmet is a spectroscopist.

This is similar to my ideas. However, I think the red shift is due to light hitting electrons (+ionized hygrogen protons) causing a Compton effect red shift.