Universe
The Universe is the summation of all particles and energy that exist and the space-time in which all events occur.
The generally accepted scientific theory which describes the origin and evolution of the Universe is Big Bang cosmology. The Universe underwent a rapid period of cosmic inflation that flattened out nearly all initial irregularities in the energy density; thereafter the universe expanded and became steadily cooler and less dense. Minor variations in the distribution of mass resulted in hierarchical segregation of the features that are found in the current universe; such as clusters and superclusters of galaxies. There are more than one hundred billion (1011) galaxies in the Universe[1], each containing hundreds of billions of stars, with each star containing about 1057 atoms of hydrogen.
Etymology
The word "universe" is derived from Old French univers, from Latin universum, which combines uni- ("one") with versus ("turn"), meaning "turned into one".[2] However, different words have been used throughout history to denote "all of space", including the equivalents and variants in various languages of "heavens", "cosmos", and "world". Macrocosm has also been used to this effect, although it is more specifically defined as a system that reflects in large scale one, some, or all of its component systems or parts.
Although words like world and its equivalents in other languages now almost always refer to the planet Earth, they previously referred to everything that exists. Some languages use the word for "world" as part of the word for Outer space, e.g. in the German word "Weltraum".
Formation
According to redshift observations, and Hubble's Law, the universe is expanding. That is, astronomers observe that there is a direct relationship between the distance to a remote object (such as a galaxy) and the velocity with which it is receding. Conversely, if this expansion has continued over the entire age of the universe, then in the past, these distant, receding objects must once have been closer together.
By extrapolating this expansion back in time, one approaches a gravitational singularity where everything in the universe was compressed into an infinitesimal point. This idea gave rise to the Big Bang Theory, which describes the expansion of space from an extremely hot and dense state of unknown characteristics.
Age
Examination of small variations in the microwave background radiation provides information about the nature of the universe, including the age and composition. The age of the universe from the time of the Big Bang, according to current information provided by NASA's WMAP (Wilkinson Microwave Anisotropy Probe), is estimated to be about 13.7 billion (13.7 × 109) years, with a margin of error of about 1 % (± 200 million years). Other methods of estimation give different ages ranging from 11 billion to 20 billion. Most of the estimates cluster in the 13–15 billion year range.