The Universe is all of space and time (spacetime) and its contents of intergalactic space and all matter and energy, which comprises galaxies, stellar systems, and planetary systems. While the size of the entire Universe is still unknown, it is possible to measure the observable universe.
- ↑ 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.""
- ↑ "Universe". Merriam-Webster Dictionary. http://www.merriam-webster.com/dictionary/Universe. Retrieved September 21, 2012.
- ↑ "Universe". Dictionary.com. http://dictionary.reference.com/browse/Universe?s=t. Retrieved September 21, 2012.
- ↑ Greene, Brian (2011). The Hidden Reality. Alfred A. Knopf.