Medieval age lasted for more than thousand years. The medieval age is formerly proclaimed to be Dark Age not because it was dark but because nothing was known.
After the collapse of Greek and Roman civilizations, the whole of Europe came under the supremacy of the churches at that which gained control. It was called as nothing was known or studied about the political structure that prevailed during that period.
Dark Age does not actually refer to the time when people spent much of their time in darkness. It simply means that nothing came into light from the period when great civilizations came to an end.