Big Bang theory:

The most important current theory for the origin of  the universe  is the Big Bang theory.According  to this theory, the universe started with'a huge explosion. It was not an explosion like the ones with which we are familiar, which start from a definite centre and spread out.  It was an explosion which occurred everywhere in space at the same time. It  filled all space from  the beginning, with  every particle  of  matter  rushing apart from every other  particle. This was  not  a  burst  of matter into  space but  rather an explosion  of  space itself. Every particle  of  matter  rushed  away from every other particle.  It  is  so far impossible to  'picture'  the  fi~st  moment  of  'creation'  of  the universe. One-hundredth of a second after the creation of  the universe is the earliest time about which scientists can speak with any confidence. At this instant, the temperature of  the universe was about a hundred billion degrees centigrade. This-is much hotter than in the centre of even the hottest star. At such temperatures none of  the components of ordinary matter, atoms, molecules, or even nuclei of  atoms, could have held together. Instead, the matter rushing apart in the explosion consisted of various types of elementary particles. The particles most abundant in the early universe were the electrons, positrons and neutrinos. There were also some protons and neutrons. The rest of  the universe was filled with energy.  It was a kind of  a cosmic soup. As the explosion continued, matter and energy rushed apart, the universe expanded and the temperatures dropped, reaching 30 billion  (3x10¹⁰)  degrees centigrade after about one-tenth of  a second; 10 billion degrees after about one second; and 3 billion degrees after about fourteen seconds. At the end of  the first three minutes, the universe became cool enough (about  1  billion ⁰C)  for the protons and neutrons to begin  to form into simple nuclei. The first to be formed was the nucleus of heavy hydrogen which was made up of one proton and one neutron. There were also helium nuclei made of  two protons and two neutrons.  It was still too hot for atoms to hold together, they were ripped apart as soon as they were created. This matter continued to rush apart, becoming  steadily cooler and less dense.  

Many thousands of years later, it became cool enough for electrons  to  join with nuclei to form atoms of hydrogen and helium. Soon, the resulting gas began  to form clumps under the influence of gravitation. These clumps ultimately  condensed to form the galaxies and stars of  the present-day  universe, almost 5 billion years after the Big Bang. There is another theory about the origin of  the universe known as the steady state theory. This theory holds that  the universe has always been just  about the same as it is now. As it expands, new matter is created continuously to fill up the gaps between  the galaxies. Thus, the problem of  the origin and early moments of  the universe is banished: there was no early universe. However, the Big Bang theory  is the most favoured by  the astronomers and astrophysicists. Why is it  so? This is due  to the evidence based on observations which lend support to the 'Big Bang' universe.