The Universe includes the Sun, Earth, solar system, galaxies and everything else that exists. Even with the help of the most powerful telescopes, astronomers cannot see the end of the Universe. Most scientists, however, believe that the Universe is mostly a vast empty space. Have you ever wondered how the Universe came into existence? There are many theories regarding the origin of the Universe. There are two main theories regarding the origin of the Universe — the Big Bang Theory and the Steady State Theory. Abbe Hamaitre put forward the Big Bang Theory in 1927. According to this theory, the entire matter in the beginning of the Universe was like a giant fireball. A big explosion took place long back and the matter of this giant fireball got scattered in all directions. On cooling down, the scattered matter became the galaxies. The matter of these galaxies has been expanding continuously. It has been estimated that this event occurred about 15 billion years ago. According to this theory, if this expansion continues, the Universe will eventually become a vast empty space. On the other hand, the Steady State Theory states that the Universe is unchanging. It has no beginning and no end. Yet another theory states that because of the force of gravitation, a time will come when the expansion of the galaxies will stop, and they will start contracting. This suggests a pulsating Universe. According to another theory, new galaxies are constantly under formation and the matter of the older galaxies goes on scattering. This is known as the theory of the balanced Universe. It is difficult to say which of these theories is reliable. However, researchers are still in search of a theory that will answer all the questions satisfactorily.