Astronomers believe that once all the matter and energy in the universe were concentrated in a single place.This place was extremely hot and dense.Then,sometime between 10 billion and 20billion years ago,an enormous and powerful explosion -The big bang -shot the concentrated matter and nergy in all directions.After the initial big bang,the force of gravity began to affect the matter racing outward in every direction,Gravity caused some of the matter to come together to form galaxies and Stars.