The Universe is all of space and time (spacetime) and its contents[1] of intergalactic space and all matter and energy,[2][3] which comprises galaxies, stellar systems, and planetary systems. While the size of the entire Universe is still unknown,[4] it is possible to measure the observable universe.


  1. Zeilik, Michael; Gregory, Stephen A. (1998). Introductory Astronomy & Astrophysics (4th ed.). Saunders College Publishing. ISBN 0030062284. ""The totality of all space and time; all that is, has been, and will be."" 
  2. "Universe". Merriam-Webster Dictionary. Retrieved September 21, 2012. 
  3. "Universe". Retrieved September 21, 2012. 
  4. Greene, Brian (2011). The Hidden Reality. Alfred A. Knopf.