The term Dark Matter Developers was coined by Scott Hanselman in this blog post from 2012. In his post he defines Dark Matter Developers:
My coworker Damian Edwards and I hypothesize that there is another kind of developer than the ones we meet all the time. We call them Dark Matter Developers. They don’t read a lot of blogs, they never write blogs, they don’t go to user groups, they don’t tweet or facebook, and you don’t often see them at large conferences. Where are these dark matter developers online?