Formation of Galaxies

Galaxies form over millions of years, making observations impossible. Scientists can see galaxies currently in different stages of development. From these observations, there are two theories about the formation of galaxies:

"Outside-in" theory. This theory states that the "halo" of the galaxy formed first. Over time matter was pulled to the center, which created the center of the galaxy. Once the center formed forces acting on it sent heavier elements throughout the galaxy. These elements, combined with hydrogen and helium, were the beginning of stars.

"Inside-out" theory. This theory is the reverse of the "Outside-in" theory. In this theory, the center of the galaxy formed first and then other clouds of gas were attracted to it. These clouds then became stars.


Source
http://www.astro.umd.edu/resources/introastro/mw.html (fair use)