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http://www.teachastronomy.com/ The big bang theory sounds as fantastic as the creation myths of many of the world cultures. How could you, and I, and the Earth, and sun, and Milky Way, and billions of galaxies have emerged from a tiny dense dot of energy and matter. There are three primary pieces of evidence. First, galaxies are all taking part in a universal expansion which manifests as a linear relationship between redshift and distance. If this expansion is traced backward it points to a time billions of years ago when galaxies were all in the same place, and the universe was much smaller than it is now. Second, the abundance of the light elements, in particular, helium, lithium, and deuterium, cannot be explained by normal fusion processes in stars but can be explained by fusion in the universe itself when it was hot young and dense. Third, space is filled with relic radiation leftover from the early hot phase which has been redshifted over the subsequent billions of years to microwaves.