What were the Dark Ages in church history?
According to this teaching, the Dark Ages were the period after the death of the twelve apostles when the guidance of the apostolic mantle was no longer at the forefront of the Body of Christ, leaving the church in obscurity with few great global moves of the Spirit.
After the last apostle died, the Church entered what is called the Dark Ages. This was the time when apostolic guidance was no longer leading the Body of Christ, and the Church went into a season of obscurity with very few great global moves of the Spirit. Since that time, the Church has grown divided, with no real structure in place to identify truth.
Before this period, the early Church seen in Scripture was guided by the instructions and revelations given to the apostles. The apostle's gift involved receiving a completed revelation of the Mystery of Christ, and pastors and elders would pour these apostolic revelations into their membership, raising up more ministers to glorify God. This was God's structure for the Church.
From the Dark Ages onward, Apostolic Order was wrongfully discarded. The Church stopped following the doctrine given by the apostles, and false doctrines were introduced into the Household of Faith. This led to a great falling away from the faith, as mentioned in 1 Timothy 4:1, and denominationalism emerged as a result. The truth concerning salvation became largely indiscernible because of this abandonment of Apostolic Order.
The scriptures
Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils;
Till we all come in the unity of the faith, and of the knowledge of the Son of God, unto a perfect man, unto the measure of the stature of the fulness of Christ:
To whom God would make known what is the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory:
For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, If ye have heard of the dispensation of the grace of God which is given me to you-ward: How that by revelation he made known unto me the mystery; (as I wrote afore in few words, Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit;
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.