Big Bang theory explains about the starting and evolution of our universe. According to the big bang theory, at the moment of big bang there was infinite energy density as well as very high temperature. But, under such unstable physical conditions we know no any atom can exist (specially, under such temperature). But after big bang happened universe started to expand rapidly through space time. With the expansion of the universe , the temperature began to decrease rapidly. After some seconds has passed the physical conditions that were in the universe allowed to create the fundamental matter particles. Nature began to create the lightest atom initially. (Hydrogen atom is the lightest atom) . Because under such relatively high temperature and under such physical conditions Hydrogen is more stable. And , decreasing the temperature went forward and forward. The forward decrease of temperature allowed to create more hydrogen atoms. After decrease the universal temperature up to some value, those Hydrogen atoms were get together and could create an atom that has more mass rather than Hydrogen atom. But the consecutive atom that has more mass rather than Hydrogen is Helium (He). And after that also universal temperature was decreasing. Creating He atoms was proceeded. After some universal time has passed, some He atoms could create more massive atoms called Li, Fe, Be and etc. And that should be the starting of the existence of galaxies, planets, stars , nebulas, quasars and surely human beings also.
Written By: Geerasee Wijesuriya