The origin of cosmic rays remains one of the greatest mysteries in Modern Astrophysics. Identifying the sources of these energetic particles indeed requires better knowledge of their spectra not only in the local interstellar medium but also at different positions in the Galactic disk. In this respect, interstellar gas is the key to reveal the mysterious origin of cosmic rays. This is because the interaction between cosmic rays and gas could result in observable signatures which might serve as indirect probes for cosmic rays. In this talk, I will first introduce two of most interesting indirect probes namely gamma-ray emissions and ionization rates together with some related anomalies concerning these observational constraints. I will then discuss the progress that has been made to better understand these anomalies and might ultimately allow us to gain more insights into the distribution of Galactic cosmic-ray sources.