The Big Bang model is a broadly accepted scientific theory for the origin and evolution of our universe. According to this theory, the universe was created about 10 billion years ago from a cosmic explosion that hurled matter in all directions. Astronomers believe that once all the matter and energy in the universe were concentrated in a single place. This place was extremely hot and dense. Then, sometime between 10 billion and 20 billion years ago, an enormous and powerful explosion — the Big Bang — shot the concentrated matter and energy in all directions. After the initial Big Bang, the force of gravity began to affect the matter racing outward in every direction. Gravity caused some of the matter to come together to form galaxies and stars. The clumps of matter around our own star, the Sun, became planets. Some planets were large enough and had enough gravity to attract matter that became moons or satellites. In 1927, the Belgian priest Georges Lemaître was the first to assume that the universe began with the explosion of a primeval atom. Years later, Edwin Hubble found experimental evidence that helped to justify Lemaître's theory. He made observations and stated that distant galaxies are going away from us in every direction at a speed proportional to their distance from the centre of the universe. Since the Big Bang, the universe has been continuously expanding, and thus the distance between clusters of galaxies has become much bigger than it was before. This phenomenon is known as the red shift. The stars and galaxies were much closer to each other in the past than they are at present, and they are currently moving rapidly away from each other. At the same time, the matter inside the galaxies continues to race away from the area where the Big Bang had occurred.Thus it has been stated that at present all the galaxies are speeding away from the centre of the universe. The Big Bang theory also predicts the existence of cosmic background radiation (the glow left after the explosion itself). In 1964, this radiation was discovered by Arno Penzias and Robert Wilson, who later won the Nobel Prize for this discovery. Although the Big Bang theory has been widely accepted, it will probably never be proved, and thus it leaves a number of tough, unanswered questions.

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