According to this theory, a large explosion aboutn ten to twenty billion years ago created the universe. As a result of the explosion, the universe became composed chiefly of radiation. Soon, the radiation formed a rapidly expanding area called the primordial fireball The main part of the fireball became matter, mainly hydrogen, after a few hundred years. In addition to the hydrogen were small amounts of helium and other light elements. The matter continued to decrease in density after the explosion. Supposedly, the matter broke apart into large clumps that formed galaxies. Within the galaxies, smaller clumps formed stars. Parts of these clumps would become solar systems.
Galaxies are currently still moving away from each other and it is probable it will remain this way until the end of time. But, it the possibility that the galaxies may collide in approximately seventy million years and explode again should not be ruled out.