The source is right above. Have you clicked on the link? They don't exactly hide the truth. If you click the link to their "400 sources", you'll be taken to a page that lists all of their "Instant Answers" sources only which supports their goal of providing information without leaving DDG. Interestingly, Google gets a lot of shit for scraping results from websites and not giving them the direct traffic, even in this very thread, but I guess we have lower expectations for a search engine that's 99% Bing.