These gas clouds are often red in color. These nebulae house stellar nurseries. The ISM contains mainly hydrogen gas. The ISM also contains significant dust which dims the light of the stars that lie behind the dust clouds. The Universe of Galaxies 1.
Irregular galaxies have no organized shape, no symmetry, contain young stars and much gas.
Galaxies Most, if not all, galaxies are in groups or clusters. Our galaxy and about 22 others are part of the Local Group of Galaxies. It is about 10 million ly in diameter. There are some truly gigantic rich clusters which contain about 1000 galaxies and are more than 20 million ly in diameter. In 1929, Edwin Hubble discovered that the Universe is expanding.