According to the much discussed and in places contested Ottoman census of 1877, there were millions of muslims spread out all over the Balkans.

Here is a map created with the data, with Kemal Karpat's Ottoman Population, 1830-1914: Demographic and Social Characteristics as primry source. Take this with a grain of salt, as Kemal Karpat's writings have been contested as sources Before. Still, regardless if these numbers are exact or not, there were a lot of muslims living all over the Balkans before the Russo-Ottoman war of 1877-1878 which created Bulgaria. While christians were extempt from conscription, they had to pay a higher tax due to the Jizya system in the Ottoman Empire, which led to a slow but steady conversion during the Ottoman rule of the Balkans from the mid-1300s until the mid-1800s.

As the Ottoman Empire lost a lot of land in the Balkans in that war, muslims fled or were ejected from their homes by the victorious powers - Serbia, Greece and Bulgaria. They fled to Ottoman Thrace, Albania and Bosnia (which all were still under Ottoman, or in the case of Bosnia, Austro-Hungarian, control), increasing the percentage of muslims in these areas.