Never heard of them.
If you've read Known Space, you'll know the foreshadowing. Remember the ships bieng pirated by the scientist with the micro black hole (that had caused the Tunguska explosion). They'd discussed the possibility of ships being eaten in hyperspace. Well, a protector theorized, then photographed, dark matter, and a dark matter creature in hyperspace.
Dark matter had never actually been discovered because it only physically exists in hyperspace, although it's gravity effects real space. And it congregates within gravity wells. Than dark matter slows ships in hyperspace down enough that the dark matter creatures can catch and eat them. And noone had ever noticed this because of the blindspot. And protectors never developed hyperdrive.
And the puppeteers knew this all along, and never told anyone. Neither did the Outsiders.