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Universe
The universe is commonly defined as the totality of existence, including planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. Definitions and usage varyand similar terms include the cosmos, the world and nature. Scientific observation of earlier stages in the development of the universe, which can be seen at great distances, suggests that theuniverse has been governed by the same physical laws and constants throughout most of its extent and history.
There are various multiverse hypotheses, in which physicists have suggested that the universemight be one among many universes that likewise exist. The farthest distance that it is theoretically possible for humans to see is described as the observable universe. Observations have shown thatthe universe appears to be expanding at an accelerating rate, and a number of models have arisen to predict its ultimate fate.

History of the universe
According to the prevailing scientific model ofthe universe, known as the 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 expanding to its present form, possibly with a brief period (less than 10−32 seconds) of cosmic inflation. Several independent experimental measurementssupport this theoretical 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 theuniverse may be in a form which cannot be detected by present instruments, called dark matter. The common use of the "dark matter" and "dark energy" placeholder names for the unknown entitiespurported to account for about 95% of the mass-energy density of the universe demonstrates the present observational and conceptual shortcomings and uncertainties concerning the nature and ultimate fate of...