(Reuters) – Investigators of a mass shooting on Saturday in Buffalo, New York, are looking into a 180-page manifesto, believed to have been authored by the teenage gunman, that outlined “The Great Replacement” conspiracy theory popular among white nationalists. The following explains its origins and significance in right-wing circles in the United States and elsewhere: WHAT DOES ‘REPLACEMENT’ MEAN? The conspiracy theory fosters the belief that leftist and Jewish elites are engineering the ethnic and cultural replacement of white populations with non-white immigrants that will lead to a “white …