The Solar System occupies just a tiny fraction of our gigantic Milky Way galaxy. There are more than 100 billion galaxies in the observable universe. They vary in shapes and sizes. The shapes are irregular, elliptical (egg-shaped), lenticular (lens-shaped), spiral, and barred spiral galaxy (bar running through the middle). A galaxy is a mass held together by its constituent parts. It is comprised of stars, planets, dust, clouds of gases, and dark matter, held together by mutual gravity. Galaxies are generally classified in a database known as New General Catalogue (NGC). Some galaxies have black holes in the galactic nucleus.