Complex sentence: The space between the stars in the Messier 87 galaxy is filled with a diffuse interstellar medium of gas, which has been chemically enriched by the elements ejected from stars as they passed beyond the end of their main sequence lifetime.
Split into simpler sentences: The space between the stars in the Messier 87 galaxy is filled with a diffuse interstellar medium of gas. This medium has been chemically enriched by the elements ejected from stars as they passed beyond the end of their main sequence lifetime.