Possible model values for f_dark as a function of [C II]/CO and volume density in the neutral atomic phase (color bar). The models shown are those corresponding to the observed ranges for N(H⁰) = [10¹⁸, 10^21.3] cm⁻², N(H₂ |CO) = [10¹⁸, 10^21.5] cm⁻², and I([CII]) = [10⁻⁸, 10⁻⁶] W m⁻² sr⁻¹. Model results are shown for a temperature in the neutral atomic phase of 100 and 50 K in the molecular phase. The striping is due to the incomplete coverage in the model results.