What is the most abundant molecule in interstellar space?

Hydrogen is the most abundant molecule in interstellar space because it is the simplest and most primordial element, formed just minutes after the Big Bang. Molecular hydrogen (H₂) is dominant in dense molecular clouds, where it plays a crucial role in star formation. It is detected primarily through infrared and radio emissions, as it lacks a permanent dipole moment for easy detection in visible spectra. The abundance of hydrogen is nearly 75% by mass in the universe, making it a key factor in chemical reactions occurring in space, including the formation of more complex molecules like water and hydrocarbons.