Large-scale structures in the Universe

The Universe has structures over various scales. Of these the ones which are of the size of a galaxy or larger are of cosmological interest and these are collectively refered to as large-scale structures. At one time astronomers were not aware of galaxies other than our own. However, with the observation of cepheid variables from outside our galaxy, it was realized that there are other galaxies too. Now we know of different types of galaxies. These differ both in their contents and in their structure. Over larger sizes we have clusters and further, super-clusters of galaxies. Observations of spiral galaxies indicate that there is a lot of dark-matter in the universe i.e. matter which does not emit light. Their presence is, however, indicated by the velocity of the stars in the galaxy. It is beleived that large-scale structures have been formed because of the gravitational collapse starting from small seed perturbations in density in an other wise homogeneous univese. The seed perturbations are produced because of quantum processes in the early universe. A number of parameters like the amount and the nature of dark-matter, density in the universe etc. govern the growth and evolution of these perturbations.