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Newly formed dwarf galaxies were likely the reason that the universe heated up about 13 billion years ago, according to new work by an international team of scientists that included a University of Virginia researcher. The finding opens an avenue for better understanding the early period of the universe's 14 billion year history. In the period of several hundred thousand years after the Big Bang, the universe was so hot and dense that matter was ionized instead of being in a neutral form. But 380,000 years later, the expansion of the universe had cooled it enough for matter to become neutral and for the first structures of the universe to form - gas clouds of hydrogen and helium. Gravity then made these gas clouds grow in mass and collapse to form the first stars and galaxies. Then, about one billion years after the Big Bang, another important transformation occurred: the universe reheated, and hydrogen - the most abundant element - became ionized for a second time, as it had shortly after the Big Bang, an event which astronomers call "cosmic re-ionization." How this happened is still debated.