The universe contains every thing that exists, from the tiniest subatomic particles to galactic super clusters (the largest structure known). Nobody knows how big the universe is, but astronomers estimate that it contains about 100 billion galaxies, each comprising an average of 100 billion stars. The most widely accepted theory about the origin of the Universe is the Big Bang theory, Which states that the universe came into being in a huge explosion-the Big Bang-that took place between 10 and 20 billion years ago. The Universe initially consisted of a very hot, dense fireball of expanding, cooling gas. After about one million years, the gas probably began to condense into localized clumps called proto galaxies. During the next five billion years, the proto galaxies continued condensing, forming galaxies in which stars were begin to born. Today billions of years later, the Universe as a whole is still expanding, although there are localized areas in which objects are held together by gravity; for example, many galaxies are found in clusters. The big bang theory is supported by the discovery of faint, cool background radiation coming evenly from all directions. This radiation is believed to be the remnant of the radiation produced by Big Bang. Small "ripples" in the temperature of the cosmic background radiation are thought to be evidence of slight fluctuations in the density of the early universe, which resulted in the formation of galaxies. Astronomers do not yet know if the Universe is "closed", which means it will eventually stop expanding and begin to contract, or if it is "open", which means it will continue expanding forever.
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