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According to the prevailing scientific model of the Universe, known as the [[Big bang|Big Bang]], the Universe expanded from an extremely hot, dense phase called the [[Planck epoch]], in which all the matter and energy of the [[observable universe]] was concentrated. Since the Planck epoch, the Universe has been [[Cosmic expansion|expanding]] to its present form, possibly with a brief period (less than 10⁻³² seconds) of [[cosmic inflation]]. Several independent experimental measurements support this theoretical [[Metric expansion of space|expansion]] and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of [[dark energy]], and that most of the matter in the Universe may be in a form which cannot be detected by present instruments, and so is not accounted for in the present models of the universe; this has been named [[dark matter]]. The imprecision of current observations has hindered predictions of the [[ultimate fate of the Universe]].

Current interpretations of [[observable universe|astronomical observations]] indicate that the [[age of the Universe]] is 13.75 ±0.17 [[1000000000 (number)|billion]] years,<ref name="marshallaugerhilbertblandford">S. H. Suyu, P. J. Marshall, M. W. Auger, S. Hilbert, R. D. Blandford, L. V. E. Koopmans, C. D. Fassnacht and T. Treu. [http://www.iop.org/EJ/abstract/0004-637X/711/1/201/ Dissecting the Gravitational Lens B1608+656. II. Precision Measurements of the Hubble Constant, Spatial Curvature, and the Dark Energy Equation of State.] The Astrophysical Journal, 2010; 711 (1): 201 DOI: 10.1088/0004-637X/711/1/201</ref> and that the diameter of the [[observable universe]] is at least 93 billion [[light year]]s, or [[Scientific Notation|8.80 {{E|x 10^26}}]] [[metre]]s.<ref name=ly93>{{cite web | last = Lineweaver | first = Charles | coauthors = Tamara M. Davis | year = 2005 | url = http://www.sciam.com/article.cfm?id=misconceptions-about-the-2005-03&page=5 | title = Misconceptions about the Big Bang | publisher = [[Scientific American]] | accessdate = 2008-11-06}}</ref> According to [[general relativity]], space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the Universe is finite or infinite.

==Etymology, synonyms and definitions==