Dark data is the inactive or unused information stored across your systems,...

https://www.cool-bookmarks.win/dark-data-is-the-inactive-or-unseen-information-companies-keep-but-don-t-use

Dark data is the inactive or unused information stored across your systems, often making up 60-80% of total data. It ties up costly storage and backup resources, and if left unmanaged, increases ransomware and breach risks