Final answer:
Giant molecular clouds are dense regions of interstellar matter found in the Milky Way Galaxy and similar galaxies. These clouds account for a significant fraction of the total mass of gas in the Milky Way and are formed in the dense regions of the spiral arms.
Step-by-step explanation:
Giant molecular clouds are dense regions of interstellar matter that are found in the Milky Way Galaxy and other similar galaxies. These clouds have densities of hundreds to thousands of atoms per cm³ and account for a significant fraction (20-30%) of the total mass of the Milky Way's gas, despite occupying a small fraction of the volume of interstellar space. The dense regions in the spiral arms of the galaxy collect the gas into giant molecular clouds, while low-density holes in the spiral arms are formed by supernova explosions.