Dark data is the inactive or unused information that sits in your systems—often...

https://www.click-bookmark.win/dark-data-is-the-inactive-or-unused-information-stored-but-not-analyzed-often

Dark data is the inactive or unused information that sits in your systems—often making up 60-80% of stored data. It increases storage and backup costs and raises ransomware and breach risks because it’s harder to monitor and protect