Have you ever wondered why you look up and see a dark sky at night? Help share science with the world: translate.minutephysics.com Subscribe to MinutePhysics! dft.ba/-MPsubscribe MinutePhysics is on Google+ - bit.ly/qzEwc6 And facebook - facebook.com/minutephysics And twitter - @minutephysics Minute Physics provides an energetic and entertaining view of old and new problems in physics -- all in a minute! Music by Nathaniel Schroeder Thanks to Nima Doroud and Niayesh Afshordi for contributions and to Perimeter Institute for support. www.perimeterinstitute.ca