The universe contain everything that exist from the thiest subatomic particles to galactic  supercluster. Nobody knows how big the universe is but atronomers estimate that it contain at least 125 billion galaxies, each comprising an average of 100 billion stars. The most widely accepted theory about the origin of the universe  is Big Bang theory, which states that the universe came into being in a huge explosion ( the big bang ) that took place between 10 to 20 billion years ago.  The universe initially consists of very hot dense fireball of expanding cooling gas. After about  one million years, the gas began to condense into localised clumps called protogalaxies. During the next five billion years, the protogalaxies continued condensing  forming Galaxies in which stars were being born. Today billion years later the Universe as a whole is still expanding although there are localized areas in which object are held together by gravity.


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