Acording to DDG's Help they don't have a cache.

Fishing for posts in various search caches is bound to be a hit-and-miss. A better way, Joe, would be for the system to log posts as they're made in a serial text file, and to email it to you automatically, every day. You keep, say, a month's worth and delete the rest. Such a log might already exist.

It could be done externally, by scraping the posts, but that requires the system to be mostly working.