>How do you know if you've never met them or talked to them?
By definition. If they tweeted or used facebook they would make their presence known and would no longer be a dark matter developer. Same with leaving comments on a blog, writing blog posts, or attending meet ups with other programmers.
I've been active on Twitter for almost 15 years, I have a couple of thousand 'followers', and I've seen posts from maybe 500,000 people at most. That's out of 450,000,000 active users at Twitter's peak. The notion that you'll have seen all, or even a significant amount, of the active devs is just daft. Same for blogs, Facebook etc. Unless Scott is open about how often he seeks out and reads blogs by people outside of his immediate bubble I think it's fair to assume this is just him failing to listen rather than millions of devs failing to speak.
Also, isn't this kind of a self-fulfilling prophecy? Once Scott sees a dev on Twitter or at a conference he must mentally move them from "dark matter" state to "not dark matter" state. By definition that means it's impossible to ever meet a dark matter dev, so they'll always be unknown to him.
By definition. If they tweeted or used facebook they would make their presence known and would no longer be a dark matter developer. Same with leaving comments on a blog, writing blog posts, or attending meet ups with other programmers.