I had never heard the term "Dark Matter Developer" before I read Geoff Graham's article on CSS Tricks. But yeah I'm one.
A "Dark Matter Developer" is one whose presence on the web or in the community is essentially non-existent.
It is acknowledged by some that it may be hard to find time for such things as user groups and social media when you're trying to get your work done and raise a family.
There's also those who find it hard to believe these developers are any good at what they do because they're not out there showing their work and they don't do any of the things that would conceivably help them become better developers.
I agree with my other dark brethren who identify as "Gray Matter", those who use the resources available online to get the job done, but just don't contribute. My wild-ass guess would be that this group makes up the majority.