A spiral galaxy is a galaxy where the stars and gas clouds are concentrated mainly in one or more spiral arms. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a concentration of stars known as "the bulge". The disk of stars orbiting the bulge separates into arms that circle the galaxy. These spiral arms contain gas and dust and younger stars that shine brightly before they dull. Most of the galaxies that scientists have discovered so far are spiral galaxies. One example of a galaxy in our solar system is The Milky Way. Spiral galaxies make up roughly 72 percent of the galaxies that scientists have observed. How the spiral arms form continues to puzzle scientists. One theory suggests the galaxy arms could be the result of density waves traveling through the outer disk. Galaxies at a whole are quite a fascination.