Why are the middle ages refereed to the dark ages?

Because they were very "Dark" times.  There was the plage (Black Death)  and many others.  There were many wars faught in that time span.  It wasn't peacefull or safe unless you were a king or in royal family.  Thats why they were referred to as the Dark ages.

they protect them in many ways.

most build a museums

give the museums money to support and finace and advertise there relgion.

cultivate talents such as cultural relics management and cultural relic protection and restoration.