Dark data is inactive or unused data that piles up in your storage—often 60-80%...

https://www.beacon-bookmarks.win/dark-data-refers-to-the-inactive-or-unused-information-stored-by-an

Dark data is inactive or unused data that piles up in your storage—often 60-80% of it. Keeping this data increases your risk of ransomware attacks and breaches because you have more sensitive information exposed