Dark data is the inactive or unused information that piles up in storage—often...

https://www.bust-bookmark.win/dark-data-is-the-information-stored-but-not-actively-used-often-making-up-55

Dark data is the inactive or unused information that piles up in storage—often 60-80% of total data—yet still creates risks like ransomware exposure and contributes to $2.5 million in annual storage and backup costs