Unveiling extensive clouds of dark gas in the solar neighborhood.

From the comparison of interstellar gas tracers in the solar neighborhood (HI and CO lines from the atomic and molecular gas, dust thermal emission, and g rays from cosmic-ray interactions with gas), we unveil vast clouds of cold dust and dark gas, invisible in HI and CO but detected in gamma rays. They surround all the nearby CO clouds and bridge the dense cores to broader atomic clouds, thus providing a key link in the evolution of interstellar clouds. The relation between the masses in the molecular, dark, and atomic phases in the local clouds implies a dark gas mass in the Milky Way comparable to the molecular one.