The Forgotten Worlds was a constellation of planets located at an unspecified point in the Galaxy. It’s moniker suggested that the worlds were somewhat isolated from the rest of civilization.
Max Dacher explanation of "crawling through the Forgotten Worlds is asking for it. That's where the predators get you." details the dangers of the seemingly lawless sector of space. Riddick's quotation of "pirates prowl the dead space, not mercs." paints of picture that the Forgotten Worlds were beyond the reach of the galactic authorities.