The Dark Ages were a low point in modern human history. They lasted for roughly a millennium from the fall of the Roman Empire in the 5th century to the Renaissance Era in the 15th century. They are often referred to as the Dark Ages because of the lack of scientific/cultural progress in comparison to the eras surrounding it. It is infamous for the Catholic Church attempting to withhold knowledge that could possibly threaten their reign over the people. This was done by using monasteries to teach the young, as the monks who lived in the monasteries were often the only people around that could read and write. Most of the time it was only the rich and powerful's children who were educated. All of this made it very easy to control what people believed, and the Church cracked down on those who differed.
In the Middle Ages, one of the biggest crimes a person could commit was heresy. If you committed this, it would make you a heretic, which is basically just someone who thinks differently. Such an offense could result in the death penalty, which at the time was being burnt at the stake. In order to determine who was a heretic, the Inquisition was formed to weed them out. This lasted from the 13th century all the way to the 18th-19th century. The Inquisition was effective in that it eliminated the opponents of the Church and struck fear into the general public, and everybody knows that the easiest people to control are the ones who are afraid. In fact, the Church was very good at controlling the masses in the Dark Ages.
The Catholic Church knew very well that if you can make people scared, they will do whatever they are told. That is precisely what they did during the Dark Ages. They put themselves in charge of education which meant generation after generation of Catholics who would do what they were told, they rejected all scientific findings of anything that could disprove what they were teaching, and formed