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 vary and similar terms include the cosmos, the world and nature.
Scientific observation of the Universe, which is believed to be at least 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, which in physical cosmology is believed to have occurred about 13.7 billion years ago.
There are various multiverse hypotheses, in which physicists have suggested that the Universe might 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 that the Universe appears to be expanding at an accelerating rate, and a number of models have arisen to predict its ultimate fate.
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