The ‘Dark Ages’ were ‘dark’ only because we lack extensive (or in some instances, any) historical material about the period between 407 AD, when the Romans marched away from Britain, and 1066, when William of Normandy conquered England.

TLP blog“Initially, this era took on the term “dark” . . . due to the backward ways and practices that seemed to prevail during this time. Future historians used the term “dark” simply to denote the fact that little was known about this period; there was a paucity of written history. Recent discoveries have apparently altered this perception as many new facts about this time have been uncovered.

The Italian Scholar, Francesco Petrarca called Petrarch, was the first to coin the phrase. He used it to denounce Latin literature of that time; others expanded on this idea to express frustration with the lack of Latin literature during this time or other cultural achievements. While the term dark ages is no longer widely used, it may best be described as Early Middle Ages — the period following the decline of Romein the Western World. The Middle Ages is loosely considered to extend from 400 to 1000 AD.”  http://www.allabouthistory.org/the-dark-ages.htm

For Wales, the time was no more or less bright than any other.  The relative peace the Romans brought was predicated on the brutal subjugation of the British people.  When the Romans left, the Britons faced the Irish from the west, the Scots from the northwest, the Picts from the northeast and ‘Saxons’ (who were Angles and Jutes too, not just ‘Saxons’) from the east.  To a certain degree, it was just more of the same.  The Britons had their lands back—the whole expanse of what is nowWales andEngland—for about five minutes.

From Gildas:

As the Romans went back home, there emerged from the coracles that had carried them across the sea-valleys the foul hordes of Scots and Picts. … They were more confident than usual now that they had learnt of the departure ofthe Romans and the denial of any prospect of their return. So they seized the whole north of the island from its inhabitants, right up to (i.e. as far south as) the wall (presumably Hadrian’s). A force was stationed on the high towers to oppose them, but it was too lazy to fight, and too unwieldy to flee. Meanwhile there was no respite from the barbed spears flung by their naked opponents, which tore our wretched countrymen from the walls and dashed them to the ground.

From contemporary accounts in 411:

Zosimus

They (the barbarians) reduced the inhabitants of Britainand some parts of Gaul to such straits that they revolted from the Roman Empire, no longer submitted to Roman law, but reverted to their native customs. The Britons, therefore, armed themselves and ran many risks to ensure their own safety and free their cities from the attacking barbarians. The whole of Armorica, [Emap (7)] and other Gallic provinces, in imitation of the Britons, freed themselves in the same way, by expelling the Roman magistrates and establishing the government they wanted. The revolt of the provinces ofBritain and Gaul occurred during Constantine’s tyranny because the barbarians took advantage of his careless government. …

Fastidius — letter to a widow in Britain

We see before us many instances of wicked men, the sum of their sins complete, who are being judged at the present moment, and denied this present life no less than the life to come. This is not hard to understand, for in changing times we expect the deaths of magistrates who have lived criminally, for the greater their power, the bolder their sins. … Those who have freely shed the blood of others are now forced to shed their own. … Some lie unburied, food for the beasts and birds of the air. Others have been individually torn limb from limb. Their judgements killed many husbands, widowed many women, orphaned many children, leaving them bare and beggared … for they plundered the property of the men they killed. But now it is their wives who are widowed, their sons who are orphaned, begging their daily bread from strangers.

http://www.cit.griffith.edu.au/~s285238/DECB/DECBbestest.html

It does seem that a ruler named Vortigern invited some Germanic ‘Saxon’ tribes to settle in eastern England, in hopes of creating a buffer zone between the Britons and the relentless invasions fromEurope.  This plan backfired, however, and resulted in the pushing westward of successive waves of ‘Saxon’ groups.  Ultimately, the Britons retreated into Wales, the only portion of land the Saxons were unable to conquer.

From the Anglo-Saxon Chronicle:

445:  In the fourth year of Vortigern’s reign, the English came to Britain.

Bede

449:  The British consulted what was to be done and where they should seek assistance to prevent or repel the cruel and frequent incursions of the northern nations. They all agreed with their king Vortigern to call over to their aid, from the parts beyond the sea, the Saxon nation. … The two first commanders are said to have been Hengist and Horsa.

Anglo-Saxon Chronicle

449:  Martian and Valentinian assumed the Roman empire(actually in 450) and reigned seven winters. In their days Hengest and Horsa, invited by Vortigern, king of the Britons to his assistance, landed inBritainin a place that is called Ipwinesfleet; at first to help the Britons, but later they fought against them.

Nennius

453:  But Hengest was an experienced man, shrewd and skilful. Sizing up the king’s incompetence, and the military weakness of his people, he held a council, and said to the British king “We are a few; if you wish, we can send home and invite warriors from the fighting men of our country, that the number that fight for you and your people may be larger.” The king ordered it be done, and envoys were sent across the sea, and came back with sixteen keels, with picked warriors in them. In one of the keels came Hengest’s daughter, a beautiful and very handsome girl. When the keels had arrived, Hengest held a banquet for Vortigern, and his men and his interpreter, whose name was Ceretic, and told the girl to serve their wine and spirits. They all got exceedingly drunk. When they were drinking Satan entered Vortigern’s heart and made him love the girl. Through his interpreter he asked her father for her hand, saying “Ask of me what you will, even to the half of my kingdom”.

http://www.cit.griffith.edu.au/~s285238/DECB/DECBbestest.html

It’s important to point out that Welsh literature, language, and culture flourished during the Dark Ages.  Much of the material in the Red Book of Hergest, the White Book of Rhydderch, and the Black Book of Camarthen date to this time.