Molecular hydrogen, H2, is thought to be the most abundant molecule in interstellar space but is difficult to detect. Which easily detected molecule occurs in space along with hydrogen, the measurement of which allows us to estimate the amounts of molecular hydrogen?

Carbon mono-oxide (CO)

Explanation:

Hydrogen exist with various molecules in space which includes carbon mono-oxide, hydroxide, ammonia, carbon (iv) oxide, silicon monoxide and other molecules. They combine in cold regions of space and exist as complex molecules. However hydrogen molecules do not emit radio energy. However CO molecules emit radio energy and hence are used to detect the presence of hydrogen.