The components of the interstellar medium primarily consist of what types of gases?

The interstellar medium (ISM) is predominantly composed of hydrogen and helium, making up about 98% of its total mass. Hydrogen is the most abundant element in the universe, forming the building blocks of stars and galaxies. Helium, while less abundant than hydrogen, still plays a critical role in the dynamics and composition of the ISM.

The presence of hydrogen and helium is essential for the processes of star formation and the evolution of galaxies. Additionally, these gases can be found in various forms such as neutral hydrogen (HI), ionized hydrogen (HII), and molecular hydrogen (H2). Helium often exists in ionized states as well, contributing to the thermal and chemical dynamics observed in the ISM.

The other types of gases listed, while they do exist in trace amounts within the ISM, are not the primary constituents. For example, even though carbon monoxide and ammonia are found in molecular clouds, they represent a small fraction of the total gas composition compared to hydrogen and helium. Hence, the correct focus on hydrogen and helium underlines their foundational role in the structure and evolution of the universe's cosmic environment.