The Dark Ages were an historical period ranging from roughly 400AD to 1066AD (from the Romans leaving Britain to the Norman invasion).It is known as the Dark Ages simply because things weren't recorded as accurately as the Romans had done, nor as the Normans would, so most historical events are a bit "shady" or "dark" as to what actually happened and where.
From a wargamer's perspective though this period is full of characterful armies and battles; the period begins with (if he existed) armies of King Arthur and other Romano-British warlords defending England against the Germanic "Sea Wolves", the Saxons, Angles, Jutes and Frisians.
The period then progresses to wars between the now settled invaders, with dynastic feuds waged between kingdoms like Northumbria, Mercia, Wessex; England being a patchwork of warlord territories.
Then in 793 a new terror arrived on the shores - The Vikings struck the holy isle of Lindisfarne on the North East coast. Raids turned into settlements and within 50 years large swathes of the country were being ruled by Viking warlords, new "Norsemen" villages appeared and other towns grew like Jorvik (York). The Saxon race fought back though, and with warrior kings like Aethelstan and Alfred the Great, the Danes were kept from over running the entire nation.
Time however brought about a synergy of cultures and the first Danish King of England, King Cnut (Canute) was crowned in 1016. A period known as Anglo-Danish followed. The army adopted a more "Viking" style, with the elite troops (Housecarls) opting for battle axes over traditional spears, within a generation the English army was one of the most respected across Europe.
It was this revered army that in 1066 King Harold marched on foot from London to York (185 miles) in just 4 days to then decisively defeat a Viking army before marching back as fast to Kent to meet the next invader - Count William of Normandy; at Hastings.