People tend to forget that after the Western Roman Empire started coming apart, the Eastern Roman Empire (the Byzantine empire centered in Constantinople) was still going strong, until it fell to Muslim invasion
It was Muslim raiders, pirates and bandits who helped KEEP Europe in the Middle Ages. Muslim pirates stopped trade in the Mediterranean, cut off contact to India and China, and generally made a feudal existence necessary to survival.
And, no, the Arabs did not invent "Arabic numerals" or algebra. That was stolen from Indian mathematicians by Arab conquerers
To be completely honest it was the 4th Crusade with crippled the Byzantine Empire and left it ripe for the taking.