The Conquerors Trilogy/Fridge


 * Fridge Logic: The Zhirrzh rule over several races that previously 'attacked' them and keep them at a low technological level, with scientists occasionally studying their planets' ecosystems and having contact with them. So in all that time none of those races ever mentioned to them that
 * Maybe they ended up slaughtering every member of each species who knew what  were. We see that at least one of those species is completely subjugated and living barely-civilized lives, not allowed to gather...