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

https://www.save-bookmarks.win/dark-data-is-the-inactive-info-your-company-stores-but-doesn-t-use-often-60-80

Dark data is the inactive or unused data stored across your systems, often making up 55% of all stored information. It matters because storing and backing up this data wastes resources and increases costs, with businesses spending millions annually