Dark data is the unused, untagged information that piles up inside your storage...

https://www.coast-bookmarks.win/dark-data-is-the-inactive-or-unused-information-that-piles-up-in-your-storage

Dark data is the unused, untagged information that piles up inside your storage systems—often making up 55% of all stored data. It raises your risk of ransomware exposure since you may not know what’s vulnerable