Are molecular clouds the heaviest objects in our Galaxy?

As radio astronomers survey our galaxy for the millimeter wavelengths characteristic of molecular rotations at low temperature, it is becoming clear that much of the interstellar gas in the galaxy is molecular rather than atomic. Furthermore, this molecular gas, mostly $H2,$ appears to be concentrated in giant molecular clouds, tens of parsecs across. With masses typically several hundred thousand times that of the Sun, these giant clouds appear to be the most massive objects in the galaxy.