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[[Image:56200main dark expansion-lg.jpg|300px|right]]'''Dark energy''' is one of two concepts (the other is [[dark matter]]) that [[Big Bang]] [[cosmology|cosmologists]] and [[astrophysicists]] have invented to explain the most serious differences to date between [[astronomy|astronomical]] observations of an [[expanding universe]] and their own expectations. It is "the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate";<ref>[http://www.encyclopediaproject.net/wiki/Dark_energy New World Encyclopedia]</ref> Astronomers and cosmologists have been speculating on the nature of this dark energy for ten years.  

[[Image:56200main dark expansion-lg.jpg|300px|right]]'''Dark energy''' is one of two concepts (the other is [[dark matter]]) that [[Big Bang]] [[cosmology|cosmologists]] and [[astrophysicists]] have invented to explain the most serious differences to date between [[astronomy|astronomical]] observations of an [[expanding universe]] and their own expectations. It is "the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate";<ref>[http://www.encyclopediaproject.net/wiki/Dark_energy New World Encyclopedia]</ref> Astronomers and cosmologists have been speculating on the nature of this dark energy for ten years.