Make a list of philosophers, writers, thinkers, playwrights, storytellers, and inventors from western civilization. Chart them out according to their date of writing. See that big gap between the end of the Roman Empire and the middle ages? That's the dark ages, and that's why it's called dark.
Some people see something fashionable about rejecting this term because relativism is 'in' these days, but it's perfectly fitting considering the reality. Some times are bright. Some are dark.