A galaxy is a huge, gravitationally bound system consisting of stars, a medium of gas and dust and dark matter. Typical galaxies range from ten million 107 stars up to one trillion 1012 stars, all orbiting a common center of mass. Galaxies can also contain many multiple stars, star clusters.
Galaxies have been categorized according to their apparent shape referred to their visual. A common form is the elliptical galaxy, which has an ellipse-shaped. Spiral galaxies are disk shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.
The formation of galaxies is the research areas in astronomy. Some ideas, however, are now widely accepted.
After the Big Bang, the universe had a time when it was remarkably homogeneous, as can be observed in the Cosmic Microwave Background, the fluctuations of which are less than one part in one hundred thousand.
The most accepted view today is that all the structure we observe today was formed as a consequence of the growth of primordial fluctuations.
The primordial fluctuations caused gas to be attracted to areas of denser material, and star clusters and stars.
One consequence of this model is that the location of galaxies indicates areas of higher density of the early universe.
Hence the distribution of galaxies is closely related to the physics of the early universe.