Big Bang Theory is one of the most important theories the beginning of the Universe. This theory is based on Hubble’s observation about the expansion of the Universe. He found that galaxies were moving apart; and farther away galaxy is, the faster it is moving away. If the galaxies are flying apart, then once upon a time they must all have been packed closely together. If we move back on time and reverse the direction of motion of these galaxies then in about 15 billion years all would condense to a mass packed together.
This dense mass called primeval mass has been estimated to be 100 million light years wide. The dense compact ass (primeval mass) was under great pressure and with temperature ranging up to trillions of degrees. The time at which this primeval atom exploded with a bang marked the birth of the Universe.
Compared to the age of the Universe which is estimated to be 15 billion years, the formation of the earth and the evolution of life on earth have occurred comparatively much more recently. The Universe is roughly three times the age of the sun ad the earth. IN order to have a comparative idea of the age of the Universe, age of the earth and the time when life evolved on it let us assume that 15 * 109 years, that is the age of the Universe is equal to one earth year. On this scale if we assume that the Universe originated on the first of January, the solar system (which includes Earth) would have been formed in the beginning of October. The human life on the earth would have evolved as late as 11:30 P.M. on the 31st of December.
What will happen in the future? Will the Universe expand forever? According to one of the theories, the Universe will keep on expanding indefinitely. As the galaxies fly part from each other, the space will grow emptier and the density of matter will decrease. Over billions and billions of years the old stars will die out one by one, the galaxies will become dim and Universe will fade into darkness.