The Dark Ages is the period of cosmic history in which there was no visible light in the universe. When the universe was about 380,000 years old, the temperature dropped to about 3,000K, electrons combined with protons to form neutral hydrogen atoms, and the radiation which had previously been scattered by the free electrons, was able to travel freely through space. The universe therefore became transparent to the passage of radiation, and this radiation was distributed over visible and infrared wavelengths. However, as the universe expanded, the radiation was rapidly redshifted to longer wavelengths beyond the visible range, (to ultimately become the cosmic microwave background radiation we detect today), and the universe became completely dark for many millions of years. These are the cosmic Dark Ages.
The Dark Ages lasted until the first stars, galaxies and quasars were created. Until recently, it was estimated that this didn't occur until the universe was 1 billion years old. Recent data from the WMAP satellite in 2003, however, suggests that the Dark Ages ended when the universe was somewhere between 100 million and 400 million years old.
The Dark Ages, or Early Middle Ages, is the period of human history between approximately AD 600 and 1000, when the classical pagan culture of Greek, Hellenistic and Roman civilisation was succeeded in Western Europe by a period of economic and intellectual stagnation.
After the Roman Empire was converted to Christianity, it fell into decline, and the Western Roman Empire was overcome by the pagan barbarians.
In 'Ideas: A History from Fire to Freud', Peter Watson writes that "in early antiquity religious toleration had been the rule rather than the exception, but that changed with the animosity with which the pagans and Christians regarded one another. We should not overlook the change that had come about in men's attitudes with the arrival of Christianity as a state religion. There was an overwhelming desire to 'surrender to the new divine powers which bound men inwardly' and 'a need for' suprahuman revelation. As a result, the thinkers of the period were not much interested in (or were discouraged from) unravelling the secrets of the physical world: 'The supreme task of Christian scholarship was to apprehend and deepen the truths of revelation.' Whereas paganism had imposed few restrictions on the intellectuals of Rome, Christianity actively rejected scientific enquiry. The scientific study of the heavens could be neglected, said Ambrose, bishop of Milan...'for wherein does it assist our salvation?'"
Between Boethius (AD c.480-524) and Anselm in the 11th century, there was only one significant thinker, Erigena in Ireland. Books and learning fell into an almost terminal decline in Western Europe, and it was the Islamic world which preserved much of the classical culture. In particular, the works of Aristotle, most of which were totally lost in Europe, were preserved by the Arabs, and only re-introduced to Europe in the 13th century.