The Big Bang Theory states that:
The observable universe began with an instantaneously expanding point, roughly ten to twenty billion years ago. Since then, the universe has continued to expand, gradually increasing the distance between our Galaxy and external galaxies. The expansion of the universe "stretches" light rays converting blue light into red light and red light into infrared light. Distant galaxies, which are rapidly moving away from us, appear redder. This expansion also cools the microwave background radiation. The cosmic microwave background radiation, which today has a temperature of 2.728 Kelvin, was hotter in the early universe. Gravity slows the expansion of the universe. If the universe is dense enough, the expansion of the universe will eventually reverse and the universe will collapse. If the density is not high enough, then the expansion will continue forever. The density of the universe will determine its ultimate fate.