Dark data is the inactive or unused information that piles up in your storage,...

https://high-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 that piles up in your storage, often making up 55% of all stored data. It increases storage and backup costs while exposing your environment to higher breach and ransomware risks