I think DDG is best considered a search broker, it federates the queries out to other engines, like bing, or others under query syntax control, and I believe they are also crawling some or their own content, although that's mostly beyond any individuals capability at this point because the internet has become quite large. The broker also merges federation sources, however it isn't a search engine in the traditional sense because the indexes (posting lists) are in google, bing, lucene etc.

* edited for autocorrect errors