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

https://wiki-cafe.win/index.php/What_Is_a_Global_Namespace_and_Does_It_Help_with_Dark_Data%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