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

https://www.first-bookmarkings.win/dark-data-is-the-dormant-info-businesses-keep-but-don-t-use-often-55-of-all

Dark data is the inactive or unused information that piles up in your storage, often making up 55% of all data. It wastes space and backup resources, driving up costs by millions annually