Why is the middle ages called the dark ages?

Which of the following is not true of the xiongnu empire? burial mounds included glass beads and mirrors made from the sands throughout the region. there was more cooperation through trade between the tribes. other tribal units that were eventually assimilated were indo aryan, xianbei, hunyu, scythina and han chinese. as each nomadic tribe and civilization was conquered or absorbed into the larger empire, the xiongnu were able to hold more territory.