Eobard Thawne, a scientist of the 25th century and fan of Barry Allen (The Flash) recreated the same reaction that gave Barry his powers. Thawne went back in time to meet his hero, but instead found out he was never gonna be like Barry, that'd he become his greatest enemy. Eobard became his opposite, his rival, he became the "Reverse" of Barry Allen. He became Professor Zoom, the Reverse Flash.