The gamma-ray sky diffuse emission comprises an anisotropic components due the interaction between the Cosmic Ray (CR) and Interstellar Medium (ISM). While dedicated survey can generally trace the abundance of specific ISM components, CRs are sensitive to the whole medium they go trough, making the CRs diffuse emission an unique trace of the total amount of matter along a given line of sight. By comparing the model with the observed gamma-ray sky, the researchers pinpoint the regions that may host large abundances of dark neutral medium (DNM). DNM may correspond with an insofar untraced phase of the ISM associated with warm atomic material recently revealed by Herschel telescope.