They do a bit of their own crawling. They take other people's crawls; Google and Bing spend a lot of money on crawling and it's odd to try to compete against those.
DDG also take a lot of other people's crawls.
DDG then add a layer on top. For example, 'official sites' are identified.
(I was reading about this just this morning, and frustratingly I cannot find the post again.)
