In the hot Big Bang Theory, 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. Thus, distant galaxies, which are rapidly moving away from us, appear redder. This expansion also cools the microwave background radiation. Thus, 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. Thus, the density of the universe will determine its ultimate fate . Since light travels at a finite speed, we see these distant clouds at an early time in the history of the universe, when it was more dense and thus hotter. In its current form, the big bang theory is not complete.