that is the question that stumped people forever. my opinion: our universe is like a 2 stroke engine,BANG it explodes sending all the nessisary elements scatterd out in an ever expanding wave. galaxys are made with suns and planets with life that live for billons of years, and the edges of the univers are still streching out further and further. after it reaches to it's full capacity it starts to retract condencing all matter and energy into it's center, untill the energy can't be held anylonger and BOOM, it explodes and we start all over again.