The energy then  might have transformed into mass, as particles could be created out  of energy.During big bang,universe was infinitely hot.As the  universe expanded, its temperature would have decreased to few  million centigrades.At this temperature, the molecules lost their  capability to overcome nuclear force,and would have started to form  atoms and molecules.This process finally gave birth to several  trillion molecules,which have been transferred evenly throughout  the space available. By this process,the universe is thought to be  originated. S Ayush