Robust knowledge of molecular hydrogen (H2) gas is critical for understanding star formation in galaxies. However, H2 is not directly observable in the cold interstellar medium (ISM) the molecular gas content must be inferred using indirect tracers like CO, total dust emission, etc. We devised a new model, which directly uses the mid infrared (MIR) rotational lines of H2 to calculate the total molecular gas content in galaxies. We apply our model in starbursting (U)/LIRGs and at very low metallicity where indirect tracers become unreliable. Finally I will conclude my talk by discussing, variation of the CO conversion factor in galaxies.