every thing was getting away from each other in the universe. also galaxies and stars were getting away too. And conclusions showed that 'the univer was expending'.
according to the this expending, if we go back in time, we could understand that the universe had began a single point. according to the conculations, a single point of universe, which is include meterial of the universe, needs to have zero volume and infinite density. the universe appeared with this point's zero volume had exploned.
this explation colds as 'big bang'
çok uğraştırdı :)