Merriam-Webster Collegiate Dictionary - dark age

 noun  Date: 1640  1. a time during which a civilization undergoes a decline: as  a. plural, capitalized D&A the European historical period from about A.D. 476 to about 1000; broadly Middle Ages  b. often plural, often capitalized D&A the Greek historical period of three to four centuries from about 1100 B.C.  2.  a. often plural, often capitalized D&A the primitive period in the development of something in the 1890s, way back in baseball's Dark Ages — R. W. Creamer  b. often plural, often capitalized D&A a state of stagnation or decline