The Autonomous Region in Muslim Mindanao (ARMM) is in the Philippines.
WARNING: A fierce armed conflict between Muslim militia and the Philippine government went on in this region from the 1970s well into the new century. In 2009 this region was declared the world's most hazardous area for journalists by the Committee to Protect Journalists, with 18 reporters dead in a massacre that claimed the lives of almost 60 people.
In 2012 a peace accord was signed between the government and the most important militia group, the Moro Islamic Liberation Front or MILF, and things have certainly been calmer since then. However, there are other militia groups that have not signed on to the peace deal; they are mostly quite small but also well-armed and quite radical, so they pose a significant threat.
Travelling to this region is strongly discouraged. If it is necessary to visit, inform your embassy, remain cautious at all times and avoid public gatherings. If you must go, see also War zone safety.
Other areas, such as the Zamboanga Peninsula, have a Muslim majority and some, such as Palawan, have substantial Muslim minorities. When the ARMM was formed these regions were offered the chance to join it, but voted not to.