A galaxy is a huge gravitationally bound system that is made out of stars and medium of gas, dust and dark matter. Galaxies differ in size and range from small dwarf galaxies with about ten million stars up to giant galaxies with one trillion stars orbiting around a common center of mass. Galaxies do not only include simple starts, but can also have many multiple star systems, star clusters and different interstellar clouds.
Galaxies are categorized historically according to their obvious shape. The most common ones are elliptical galaxy with ellipse shaped profile, spiral galaxies with disk- shaped, curving and dusty arms profiles. Not all galaxies have clear shapes and those ones are called peculiar galaxies and their different shape is cause due to the gravitational pull of the neighbor galaxies. The space between galaxies is filled with tenuous gas.
Astrologists have discovered more than 100 billion galaxies in the observable universe. Most of the galaxies are organized into clusters, which can form superclusters or simply larger groups. It is believed that dark matter makes out 90% of the mass of the galaxies. Furthermore, huge black holes are believed to be in the center of many of the galaxies, some even believe that in most and they are the reason for active galactic nuclei found in the galaxies.
