Featured VIDEO: Carl Sagan "Pale Blue Dot"

The Universe is by definition everything that physically exists in space and time. It originated in the Big Bang 13.7 billion years ago. The radius of the Universe is at least 78 billion light years. That is the distance to the point from which a photon of light originated which is now reaching us after traveling for 13.7 billion years. However the Universe could be far larger. There is no limit to the speed at which space itself may expand, so the Universe may stretch far beyond the Hubble horizon expanding away from us faster than the speed of light. It is also conjectured that our Universe may be only one of 10⁵⁰⁰ other universes which may have totally different laws of nature and which are collectively called a multiverse.