The Universe is the totality of existence. This includes planets, stars, galaxies, the contents of intergalactic space, the smallest subatomic particles, and all matter and energy, the majority of which are most likely in the form of dark matter and dark energy.
The part of the Universe that we can see, referred to as the observable universe, is about 28 billion parsecs (91×109 ly) in diameter at the present time. The size of the whole universe is not known and may be infinite.Scientific observation of the Universe has led to inferences about its evolution. These observations suggest that the Universe has been governed by the same physical laws and constants throughout most of its extent and for all time. The Big Bang theory is the prevailing cosmological model that describes the development of the Universe. Assuming that the prevailing model is correct, the age of the Universe is measured to be 13.798 ± 0.037 billion years. Space in the Universe is expanding, and the rate of its expansion is increasing.
There are many competing theories about the ultimate fate of the Universe. Physicists remain unsure about what, if anything, preceded the Big Bang. Many refuse to speculate, doubting that any information from any such prior state could ever be accessible. There are various multiverse hypotheses, in which some physicists have suggested that the Universe might be one among many universes that likewise exist.
The word universe derives from the Old French word univers, which in turn derives from the Latin word universum. The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used. The Latin word derives from the poetic contraction unvorsum — first used by Lucretius in Book IV (line 262) of his De rerum natura (On the Nature of Things) — which connects un, uni (the combining form of unus, or "one") with vorsum, versum (a noun made from the perfect passive participle of vertere, meaning "something rotated, rolled, changed").
Throughout recorded history, cosmologies and cosmogonies have been proposed to account for observations of the Universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers and Indian philosophers. Over the centuries, more precise observations led to Copernicus's heliostatic model of the Solar System, and Kepler's heliocentric and elliptical model of the Solar System. The concept of gravity led to the Newtonian model of the Solar System. Further improvements in astronomical observations led to the realization that the Solar System is located in a galaxy composed of billions of stars, the Milky Way. And, then, it was subsequently discovered that our galaxy is just one of many. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. The discovery in the early 20th century that galaxies are systematically redshifted suggested that the Universe is expanding, and the discovery of the cosmic microwave background radiation suggested that the Universe had a beginning