Dark data is all the information your team stores but never uses—about 55% of...

https://zandersexcellentblog.quillnesty.com/posts/why-is-our-backup-taking-longer-every-month-unpacking-the-impact-of-data-sprawl-and-dark-data

Dark data is all the information your team stores but never uses—about 55% of it sits unused. This inactive data still costs money to keep and can create security risks