Dark data is all the inactive or unused info storing away in your company...

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

Dark data is all the inactive or unused info storing away in your company systems—think 60% to 80% of data sitting idle. It’s not just taking up space; it drives up costs and can cause security risks