What materials make up most nebulae in the Milky Way Galaxy?

Most nebulae in the Milky Way Galaxy are composed of interstellar matter, which consists primarily of gas accompanied by small solid particles known as cosmic dust.

The chemical composition of these nebulae is relatively uniform and reflects the general composition of the universe. By atomic count, approximately 90 percent of the constituent atoms are hydrogen, while nearly all of the remaining atoms are helium. Other elements—such as oxygen, carbon, neon, and nitrogen—together account for only about two atoms per thousand.1

While gas makes up the vast majority of the mass, dust represents about 1 percent of the mass of the interstellar medium. Despite this small fraction, these solid grains are significant because they are highly efficient at absorbing and scattering radiation.2