The amount of dark social that comprises one's direct traffic is going to vary. For example, you might be diligent about incorporating tracking tokens into your email marketing campaigns, and asking your co-marketing partners to do the same when they promote your site content. Great. You won't have a huge chunk of traffic dumped into direct traffic that should really go into email campaigns. On the other hand, perhaps you're embroiled in a viral video firestorm, and a video on your site gets forwarded around to thousands of inboxes ... you don't exactly have control over that kind of exposure, and as a result you'll be seeing a lot of traffic without referral data in the URL that, consequently, gets bucketed under direct traffic. See what I mean? It depends.
The term “organic traffic” is used for referring to the visitors that land on your website as a result of unpaid (“organic”) search results. Organic traffic is the opposite of paid traffic, which defines the visits generated by paid ads. Visitors who are considered organic find your website after using a search engine like Google or Bing, so they are not “referred” by any other website.