The 'dark ages' are called the 'DARK ages' for a reason, They started with an event that, literally, put the earth into darkness. Something was blocking out the sunlight. It is hypothesized that it was a volcano that created a thick dust cloud around the earth. See [The Dark Ages Were Caused By Two Enormous Volcanic Eruptions](https://motherboard.vice.com/en_us/article/ezpv7n/the-dark-ages-were-caused-by-two-enormous-volcanic-eruptions) and [Climatic and societal impacts of a volcanic double event at the dawn of the Middle Ages](https://link.springer.com/article/10.1007%2Fs10584-016-1648-7) for a scholarly article.

> Volcanic activity in and around the year 536 CE led to severe cold and
> famine, and has been speculatively linked to large-scale societal
> crises around the globe.

In London, England, the 'pea soup' fogs that blocked out the sunlight were due to particulate pollution from burning dirty coal fires.

So there are a few possibilities in your world.

I would suggest that there are huge coal or other organic or carbon deposits, that have been set on fire and have been burning for a very long time. These would produce a huge particulate smoke cloud that covered an area.

Your question does not specifically state that this has to be planet-covering, just covering 'your world'. During the Dark Ages, their 'world' was Europe. It also does not give a time frame for this effect.

A second option would be continuous dust storms. A weather pattern that produced constant high velocity upper atmosphere wind patterns. The wind energy would have to come from some extreme temperature differential. Perhaps a tidally locked planet with a thick atmosphere would produce constant convection air currents from the hot side to the cool side. Since the rotation of the planet would be very slow, there would be no or minimal Coriolis effect.

A third option would be continuous volcanic activity on the planet. An earthly example would be [Two centuries of continuous volcanic eruption may have triggered the end of the ice age](http://theconversation.com/two-centuries-of-continuous-volcanic-eruption-may-have-triggered-the-end-of-the-ice-age-83420). Also, apparently Io has permanently active volcanoes.