As Planet Discoveries Pile Up, a Gap Appears in the Pattern
Ok, so we can explain this plus Dark Matter in one shot.  Basically, planets slightly larger than Earth are practically guaranteed to evolve intelligent life.  Intelligent life eventually develops technology that allows them to shield the existence of their planet, or even entire stellar system, from the probes of the Evil AI Lifedestroyer.  So, all that Dark Matter is just the intelligent life in the universe, using normal prudence to keep out EAILD (see above), more or less like most humans live in houses with walls.<p>So, any system that is not part of Dark Matter yet is either:
1) unconducive to life (or complex life, anyway)
2) was already zapped by the EAILD
3) like us, is in a transient state where the EAILD hasn't arrived yet, but we haven't learned how to cloak our solar system yet either<p>Again, the key here is that planets just slightly larger than Earth are the most likely to evolve intelligent life, so they nearly always end up cloaking, and thus we don't see them.<p>Enjoy.