But I got to the bottom of it at last. So we look back into history, although it's somewhat cloudy. When the Ottoman Turks took over Istanbul, they decided to use the town's symbol as their own. Other explanations, however include the story that Osman, founder of the Ottoman Turkish Empire in 1295, had a dream of a crescent moon stretching over the earth, took it as a good omen, and adopted it as the symbol of his dynasty. Another story says the crescent and star originated in central Asia, brought to Istanbul by the Ottoman Turks.
Some sources suggest it was originally a crescent moon and sun.
Regardless of earlier origin, by the mid-1300's it was the symbol of the Ottoman dynasty. Twas not so, but it's not the first time that erroneous understandings have taken root. So the bottom line is that the symbol of crescent and star technically were the symbols of the Ottoman dynasty, but have come to be thought of as symbols for Islam, even by Muslims. There is some debate in the Muslim community even today about the use of the crescent. Some believe that a symbol helps to distinguish Muslim establishments from non-Muslim establishments, especially from Christian places that have crosses and crucifixes everywhere.