The Universe is defined as all forms of matter, energy, space and time in its entirety. The universe is estimated to be around 13.7 billion years old, and having an observable diameter of at least 93 billion light years containing more than 100 billion galaxies. Currently, the Big Bang theory is the accepted explanation of the beginning of the universe. It states that the universe started with a cosmic explosive event, called big bang, and it has been expanding at an accelerated rate since then.