Dark data is the unused information stored in your systems—often 55% or more of...

https://direct-wiki.win/index.php/Why_Keeping_Dark_Data_on_Flash_Storage_Is_a_Bad_Idea

Dark data is the unused information stored in your systems—often 55% or more of all enterprise data. It can increase storage and backup costs by millions annually and raises ransomware and breach risks since it’s rarely monitored