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

https://record-wiki.win/index.php/Why_Does_Dark_Data_Make_Ransomware_Recovery_Slower%3F

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