One, you kind of touch on, when you talk about email on “dark matter”. But mailing lists are still a lot bigger and more pervasive than person-person email, and a lot of them are not all that dark. There are mailing list archives going back to the late ’70s.

The other is about as un-dark as you can get, and thanks to Google it’s eminently trackable from the web… and that’s Usenet. This goes back almost as far as mailing lists and it’s far better archived, cross-referenced, and like mailing lists it’s “push” technology: it reaches out and touches people rather than waiting for them to come and find it.