There wasn't one. It's propaganda. A myth.
There was a pretty standard medieval conquest. It's not like the world was peaceful back then and had the Mongols not invaded everything would have been sweetness and light.
Local sources tend to identify the invaders as Turks rather than Muslims which is a bit strange if they were carrying out a religiously motivated genocide. Timur also saw himself as representing Ghengis Khan's legacy, not that of the Early Islamic conquests.
PBS has an excellent documentary called "Empire of Faith" on YouTube. Anyone who wants to trash Islam and claim it wasn't a tolerant society by even modern standards, and an advanced one far ahead of its time at that, needs a wake up call.
It was, even as Europe wallowed in its own filth just to the north of Muslims Spain, with its endless killings, Inquisitions and oppression into ignorance of Europe called the Dark Ages, Muslim Spain had Christians, Jews, Muslims and anyone in the world who wanted an education either went their or Baghdad.
Use of the word tolerant is a bit anachronistic for any medieval society (especially a conquest oriented one) as the concept is decidedly modern.
By modern standards it certainly wasn't tolerant though as non-Muslims were, on the whole, jizya paying 2nd class citizens. Numerous churches were demolished or converted into Mosques (such as that of St Vincent which was replaced by the Grand Cordoba Mosque) Women, like elsewhere at that time, were not equal. Slavery was normal part of society, etc.
Also the name dark ages relates to the relative lack of historical texts from the period, and it was nowhere near as backward as you are presenting it although the decline of the Roman Empire and the rise of the Islamic Empire did have negative economic effects on Europe as it made international trade more difficult which compounded the obvious social upheaval of the end of Roman rule.
