What is Anatomy of the Universe

The Universe contains everthing  that exists, from the tiniest subatomic particles to galactic superclusters ( the largest structures known ). Nobody knows how big the universe is, but astonomers estimates that it contains 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 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 billions years ago . The Universe initially consisted  of a very hot , dense fireball of expanding , cooling gas. After about one million years ,the gas began to condense into localized clumps called  Protogalaxies . During the next five billion years , the protogalaxies continued condensing, forming galaxies in which stars were being 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 discovery of faint , cool back ground radiation coming evenly from all directions. This radiation is believed to be the remnant of the radition produced by the Big Bang . Small  ripples in the temprature of the cosmic background radiation are thought  to be evidence of slight fluctuations in the density of early Universe , which resulted in the formation of galaxies. Astronmers 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.