They don't call it "empty space" for nothing. No matter how many of these rogue planets/dark stars there are wandering about space, its highly unlikely we are in danger of being affected by any. How do we know this? Well, there hasn't been any around for the last few million years, right? And if there were any in the distant past, the Earth seems to have survived all right.

The close encounter with one of these dudes we're most likely to have will probably be a near miss--one will fly through the Oort Cloud and shake a few comets loose to rain down on the inner solar system. It would take centuries to play out, and, assuming our current level of astronomical knowledge, we'd be well aware of what was going on. In fact, this isn't even a new conspiracy theory, check out the "Nemesis" bogeyman that was circulating a few years ago.