That has been my impression too. Dark matter developers are people who work in software development in the employ of a company. Their employer may - and usually does - have a blog and a twitter and may well have dedicated marketing and sales teams. So even though these developers may not personally market, their employment depends upon these non-dev activities.

For me, personally, blogging and going to conferences is pure marketing. It works too. It results in me and the guys who work with me to have an income, thankfully. It's about demand creation for our services. Many people who are awesome computer scientists and programmers are not looking for that lifestyle. They work for good companies that pay them well to code. They do not have to worry about the non-coding tasks that need to happen for there to be that coder job.

Sometimes I envy their situation, but I am now personally addicted to self-employment, business, and startups. I love coding and do it for my livelihood, but I do not feel that I am called to work as just a programmer for a company.

I do like the analogy still. It's like the dark matter halo that presumably encircles our galaxy. It's unseen but is vitally important for all of our existence.