The Universe is believed to be in the region of 13.7 billion years old and is made up of planets, stars, galaxies, the contents of intergalactic space, and all matter and energy. Scientific observation of the Universe, the observable part of which is about 93 billion light years in diameter has led to inferences of its earlier stages. These observations suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and history. The Big Bang theory is the prevailing cosmological model that describes the early development of the Universe.
According to the prevailing scientific model of the 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 “Cosmic_expansion” to its present form, possibly with a brief period 10−32seconds) of cosmic inflation. Several independent experimental measurements support 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 the Universe 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 entities purported 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 the Universe. (Wikipedia)