Galaxies and the Observable Universe The universe that we are a part of began as a singular event -- one we call the Big Bang -- that has been measured to have happened 13.8 billion years ago. From the energy and matter created in this event, first stars were formed, and then galaxies of stars. A galaxy is a system of millions or billions of stars, gas and dust held together by gravity in space. There are three main types of galaxies in the universe: 1) Spiral Galaxy -- A galaxy with a flat, spiral shape: Our galaxy, the Milky Way, has a bar shape in the middle of it, and is thus called a barred spiral galaxy: 2) Elliptical Galaxy -- A galaxy with a ellipsoidal or spherical shape: 3) Irregular Galaxy -- A galaxy with no defined shape: Small and Large Magellanic Clouds -- Two irregular galaxies close to the Milky Way: There are hundreds of billions of galaxies in the observable universe. They are spread out in a structure resembling the web of a black widow spider: Credit: Sloan Digital Sky Survey Since its beginning, the universe has been expanding. Today, the observable universe is estimated to be about 93 billion light years in diameter. That is, about 46 and half billion light years in any direction from Earth. This limit is not determined by the limits of what our telescopes can see. But rather, because at the outer edges of what we can measure, the universe is expanding away from us faster than light can reach us! Thus, we really don't know how big the universe is. Credit: Andrew Z. Colvin on Wikipedia .