Why there is a galaxy?

Answer 1

A galaxy is a cluster of stars, nebulae, dark matter, and other astronomical objects. Most galaxies are tens of thousands of light years in diameter, and contain billions of stars. Galaxies come in three primary shapes; spiral galaxies are thin disks, with spiral arms surrounding a central hub; elliptical galaxies are uniform, oval-shaped agglomerations; and irregular galaxies have little or no definite structure.

A spiral galaxy centers around a hub, which is roughly spherical and bulges outward from the disk. The hub is the gravitational center of the galaxy, and usually contains a supermassive black hole. Surrounding the hub are spiral arms, which are bunched-up waves of stars and gas orbiting the center. The spiral arms usually have more gas and dust, and they contain most of the newly formed stars, which make them appear blue in color imagery. A spiral galaxy may have a horizontal bar passing through its hub; our own galaxy, the Milky Way, is believed to contain a bar.

Answer 2

First, there is not "a" galaxy (i.e., one galaxy), there are at least a hundred billion galaxies in the observable Universe!

The reason galaxies exist is basically gravity - gravity will cause mass to clump together.