The universe includes the Sun, Earth, solar system, galaxies and everything else that exists. Even with the most powerful telescopes, astronomers can not see the limit of the universe. Most scientists believe, however, the universe is mostly a vast empty space.
Now the question that arises is how did this universe come into existence? There are many theories regarding its origin. According to one theory, the entire matter in the beginning of the universe was like a fire-ball. At a certain time, a big explosion took place and the matter of this giant fire ball got scattered in all directions. On cooling down these scattered parts gave birth to the galaxies. The matter of these galaxies has been expanding continuously.
This incident took place around 20 billion years ago. Obviously, if this expansion continues, it will make the universe empty.
However, according to another theory, 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. Again, according to a third theory, new galaxies are constantly under formation and the matter of the older galaxies goes on scattering. This is the theory of the balanced universe. It is difficult to say which of the three theories is authentic. However, in order to propound a correct theory regarding the origin of the universe and its existence, researches are still in process.