The Dark Ages
The expression 'the Dark Ages' - always with the definite article and in the plural, and with initial capitals for 'Dark' and 'Ages' - used to be used to refer either to the period of European history which extends from the end of the fifth century CE to the middle of the fifteenth century, i.e., c500 CE-c1450 CE, (the period more commonly referred to as 'the Middle Ages') or to the first half of this period, i.e., c500-c1000 CE, (the period usually referred to as 'the Early Middle Ages'). Nowadays the use of the expression in either way is frowned on by historians, and you are strongly advised not to use it.
Historians from the seventeenth to the nineteenth centuries used the expression 'the Dark Ages' to refer to the period from the end of the Western Roman Empire in 476 CE to the Renaissance in the fifteenth century because they considered that those who lived in this period, lacking as they did the learning and culture which characterised Classical Antiquity and which was rediscovered in the Renaissance, lived in a state of 'darkness'. The use of the expression was also thought to be appropriate because it reflected the relative lack of material about this period available to historians by comparison with the wealth of material about Classical Antiquity and the centuries since the Renaissance. In the nineteenth century, however, as more material became available, historians came to understand the achievements of those who lived in the Middle Ages and, with this, the inappropriateness of characterising the period as one of 'darkness'. They began to use the expression 'the Dark Ages' to refer only to the first centuries of the Middle Ages, i.e., the period from c500 CE-c1000 CE, and with the further progress of historical scholarship in the twentieth century, the use of the expression to refer even to this period is now felt to be inappropriate.
See further The Middle Ages.