Most of the lands overrun by the Muslims in the “Golden Age of Islam” were Christian...North Africa and the Middle East. Persia is probably the most significant exception. The Muslim attack on what we call “India” today came much later (beginning at the end of the 12th century - and it was genocidal in its ferocity).
As for achievements, you seem to have missed the word “mainly”. The point was, and is, that Islam has always been culturally parasitic, but a parasite that ultimately destroys its host culture.
Here is a link to an article by a historian at Northwestern that you might find interesting: http://www.mmisi.org/ir/41_02/fernandez-morera.pdf
You may also find this of interest (a letter from an Assyrian software entreprenuer attempting to educate Carly Fiorina): http://www.ninevehsoft.com/fiorina.htm
Finally, the phrase “the Dark Ages” was a propagandistic phrase coined by the poet Petrarch to exalt classical antiquity in the service of the Renaissance. Serious historians understand that the Middle Ages were far from dark and produced a great deal of technological progress, among other things. The demise of the Western Roman Empire was hardly a bad thing.
It literally became dark, all sunsets were red for many decades afterward, plant growth withered in Northern and Western Europe, and a good deal of the event was reported in the British chronicles of that day (SEE: King Arthur, et al).
Europe's economy collapsed for roughly the next 500 years, and China's for the next 300 years. Things weren't all that good for several centuries later either.
All you need to understand how a bunch of un-educated guys dressed in last year's rags were able to move out of Arabia and conquer Damascus is note that even Byzantium hadn't paid its troops in over a century when the legions met the guys from Mecca.
The early Moslems did most of the conquering with Byzantine armies and Byzantine navies ~ who they paid!