Dark data is the inactive or unused information your company stores, often...

https://future-wiki.win/index.php/What_Is_KAPPA_Data_Services_and_Why_Is_It_Tied_to_Python%3F

Dark data is the inactive or unused information your company stores, often making up 55% of all data. This unused data not only wastes storage and backup resources but also increases exposure to ransomware and breach risks