   It is the whole of existence, including planets, stars, galaxies, the contents of intergalactic
    space, and all matter and energy.
   The study of universe is called as Cosmology.
   The age of universe is approximately 13.8 billion years (13.8 x 109 years).
   The size of the Universe is unknown; it may be infinite.
   The observable universe is about 46 billion light years in radius.
   It includes billions of galaxies. But there are probably more than 100 billion (1011) galaxies
    in the observable Universe.
   The present overall density of the Universe is very low, roughly 9.9 × 10−30 grams per cubic
Composition of Universe
 The Universe is believed to be mostly composed of dark energy and dark matter, both of which
  are poorly understood at present. Less than 5% of the Universe is ordinary matter, a relatively
                                          small contribution.
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   Aristotle, a Greek philosopher, state that the Earth was the center of the Universe and the
    Sun, the Moon, and the other planets are orbited the Earth. This is the "geocentric theory."
   Aristarchus was first to propose that the sun is the center of universe.
   Claudius Ptolemy (in 140 AD) proposed the theory that the earth was the center of
    universe and the sun and other heavenly bodies revolved around it.
   Nicholas Copernicus (in 1543 AD) stated that the sun is the center of universe.
   Johannes Kepler stated that the sun is the center of solar system and not the universe.
   The existence of galaxies was fist demonstrated by Edwin Hubble in 1924.
Evolution of Universe
The three main theories put forward to explain the origin and evolution of the universe are: