Universe can be defined as the totality of existence, which includes stars, galaxies, planets, intergalactic space, from quarks to quasars, atoms, energy and matter. So everything that we can observe and see is universe. The term universe can also be used in slightly different background senses, representing such impressions as the cosmos, the world, or nature.
Observable universe, so far is about 46 billion light years in radius. The universe is governed by physical laws and constants. Observation of supernovae has indicated that the universe is expanding. We translate past of universe with the Big Bang Theory and future of the universe can be explained by many competing theories, but we cannot believe on one theory depending on our present technology skills. One theory about the fate of the universe is the Multiverse or parallel universes. In this theory scientists explain our universe as a part of many others. Cosmology is the subject that tries to explain past, present and future of the universe.
Man has tried in many ways to understand the universe from the beginning. Many computable geocentric models were established by the ancient Greek philosophers. More detailed explanations and improved theories of gravity led to Copernicus's heliocentric model and Newtonian solar system model. Further research showed that the solar system lies in a galaxy. And a galaxy is composed of billions of stars. Many galaxies exist outside our Milky Way Galaxy. Careful study of these galaxies and their spectral lines led towards modern cosmology. Discoveries like red shift and cosmic microwave background radiation proposed that the Universe is growing and once had a beginning. Many models helped us for explaining the universe and one of the important breakthroughs was Einstein’s theory of relativity. This theory developed a new sense of understand time and space.
Since the weakest photon of light to the largest cluster of galaxies eternally gathered, the laws that everything in this Universe obey are the same. This is the most amazing thing about our universe. Fundamental laws of the Universe are same everywhere. Most scientists believe that the answer to the question “how did universe came into existence”; is that the Universe bounced into existence from a singularity. The singularity is a term physicists use to define states of space that disobey the current laws of physics. We believe that other singularities may exist in the cores of black holes. The Universe expanded from very dense, very small, and very hot to the cool expanse that we can observe today. This theory is now called as the Big Bang Theory; this term was first coined by Sir Fred Hoyle in a British Broadcasting Corporation (BBC) radio transmission in 1950. Most scientists suggest that it was not an explosion as the name propose, but was a rapid expansion of space and time. It can be compared to blowing up a balloon, as you blow air in the balloon and the outside of balloon expand outward.