Dark data is the inactive or unused information that builds up in your...

https://www.adirs-bookmarks.win/dark-data-is-the-unused-information-stored-in-your-systems-often-making-up-55

Dark data is the inactive or unused information that builds up in your storage—often 60-80% of what you keep. It increases your risk of ransomware attacks and data breaches because it’s overlooked in security scans