The Universe is the full expanse of space and everything in space. Literally everything that exists is included in the universe. The universe is expanding, and we know this because scientists are observing galaxies distancing more and more as time goes by. The speeds that they are moving are proportional to their distance away from us. The cosmological principle tells us that physical laws that apply to our part of the universe applies to all. This tells us that our universe is homogenous and isotropic. Homogenous means that the universe is generally the same in all places. Isotropic means that you can see the same conditions in all directions of the universe. The universe is made up of hundreds of billions of galaxies, which are collections of stars, gas, and dust. Our Sun is located in the Milky Way galaxy. The Milky Way is not the center of the universe; the universe has no center. Galaxies are most commonly classified by their shape; they can be considered spiral, elliptical, irregular, or S0. Spiral galaxies are flattened and thin like a disk. Elliptical galaxies are more oval or football shaped. Irregular galaxies are neither spiral nor elliptical. S0 galaxies appear to be a cross between a spiral and elliptical galaxy. Galaxies are found in groups or cluster; they are not all the same size. The groups are gravitationally bound and they orbit each other. Our galaxy, the Milky Way, is in the Local Group.