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

https://iris-wiki.win/index.php/Warm_Cutover_vs_Big_Bang_Migration:_Which_Is_Less_Risky_for_Your_Data_Migration_Plan%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