The Structure of the Universe
The view of the universe reveals a hierarchical system of structures. Stars are grouped in giant star clusters: galaxies, involving the most prominent star formation regions in their molecular clouds, and stellar associations such as open and globular clusters. A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter. Galaxies form clusters of progressively growing size, while galactic clusters form much larger superclusters of galaxies which are not uniformly distributed in the universe. This nonuniform distribution of clusters of galaxies forms a more ordered structure composed of walls and voids, which is called the Cosmic Web and is regarded as the remnants of fluctuations in the matter of the expanding universe after its origin (the Big Bang).