These dark matter apps include internal projects, projects under development or projects we don’t know about. We have hints that these are out there though. For instance, there’s a recent post about a XUL developer position for a lab management project that’s not released yet*.
* There’s a bug open about creating a better consultants page and this could make it easier to match Mozilla developers with people looking to build Mozilla-based apps. If anyone is interested in helping with this, let me know.