Interstellar matter refers to the gas and dust that exists in the space between stars in a galaxy. This material is primarily composed of hydrogen and helium, along with trace amounts of heavier elements. It plays a crucial role in the formation of new stars and planetary systems, as it can collapse under gravity to create dense regions that eventually ignite nuclear fusion. The study of interstellar matter helps astronomers understand the lifecycle of galaxies and the processes that lead to star formation. Observations of this matter can reveal information about the chemical composition of the universe and the conditions that existed shortly after the Big Bang.