Giant molecular clouds are among the largest objects in our Galaxy. Why, then, were they discovered only relatively recently?

A giant molecular cloud (GMC) is a type of interstellar cloud that is made up of gas and dust. The 'molecular' in GMC refers to the fact these clouds are composed predominantly of molecules, as opposed to atomic or ionized material. These clouds are among the largest entities in our galaxy and are the primary sites of star formation.

Observations in astronomy are not limited to what can be seen in visible light. The entire electromagnetic spectrum, from radio waves to gamma rays, is used in astronomical observations. However, in the past, there were severe limitations to these observational techniques. Up until recently, observational astronomy largely depended on optical telescopes that worked within the visible light spectrum.

The reason why GMCs were discovered relatively recently is due to their characteristics and the way astronomical observations used to be done. The gas and dust in GMCs are very cold, barely above absolute zero. Therefore, they emit energy primarily at radio and infrared wavelengths instead of in visible light. These clouds are also very dense, blocking light from objects inside or behind them. Since conventional optical telescopes operating in the visible light spectrum were major tools for astronomical observations until the late 20th century, GMCs, which are virtually 'invisible' in visible light, remained undiscovered until techniques and instruments capable of observing at other wavelengths, particularly radio and infrared, became prevalent.