According to the Church of the Flame, The Outer Dark is a realm that is populated entirely by demons. It is the source of the street demons Uma Shusha and the Ifrit, which hunt and feed on the urchins of Makal.
Church doctrine dictates that the Lord of the Flame imbued humanity with warmth and passion in an effort to push back the cold and darkness of the Outer Dark. It is this warmth that attracts the malicious demons to the physical realm.