Galaxies have been viewed as the building blocks of our universe since the 1930s. Before that time, galaxies were a subject open to speculation. Wonder at the universe has always provoked theories about how our universe and the galaxies it held came to be.
Understanding how galaxies are made requires revisiting the big bang theory. The big bang theory has been seen as controversial in the past, but is now accepted as the most prevalent cosmological theory of the birth of the universe. Within the past ten years, Hubble technology has made it possible to achieve surmounting evidence that the big bang theory holds the answers to the formation of galaxies.
Between a time period of ten to fifteen billion years ago, the universe was a hot, dense area of gas and matter which expanded due to increasing heat. During this burst or expansion, matter and gasses were pushed away from the center. Galaxies are gas formations that have resulted from the big bang. The universe has been expanding since that amount of time.New galaxies are still being formed as others are being altered.
Several different types of galaxies exist according to how and when they were formed. The two basic galaxy formations are spiral and elliptical. Spiral galaxies are known as one of the most distinctive types of galaxies. As the name suggests, spiral galaxies are spiral disks with a bright center known as a nucleus. Spiral galaxies hold a special significance as The Milky Way is in a spiral formation. Spiral galaxies are believed to be formed from the collapse of a photo-galactic cloud of gas.
Elliptical galaxies are thought to be formed from two disk-shaped galaxies merging as one. When two spinning spiral galaxies emerge, the orbit of their stars becomes random. Rather than the stars circling around a nucleus, or center, their movements are independent. This gives an elliptical galaxy its flat, scattered appearance.
Galaxies change shape over time. Just as spiral galaxies collide with each other, all small galaxies may collide with each other, depending on the rate at which the universe expands. Clusters of smaller galaxies hover near each other making transformations inevitable. Larger galaxies formed of smaller ones may become super galaxies, which will not easily collide with other super galaxies.
In this sense, the super galaxies, which were the first thing formed in the universe, may also be the last thing to die.
Sources
http://www.universetoday.com/1359/some-galaxies-are-made-almost-entirely-of-dark-matter/
http://www.astronomynotes.com/galaxy/s10.htm
