Ah, thanks. Thats explains it quite good. Basically everywhere in space we have at least some energy. And where we have enery particles will pop into existance and be destroyed again (photons mostly I guess for low energy like with the background radiation in space). These particles only live a short amount of time, but they can still collide with objects like dust or even planets. And when these objects move, the particles in the front will basically collide harder/more, than in the back. Thus a small force against the moving direction is exerted on the object, proportional to its speed (just like friction). Though this is only significant for small objects like dust or on a very very very long time scale.