Nearly 500 years ago, the Angels of Heaven 'fell', cast out by God. The Humans, so full of sin, no longer had a place beside God. The Angels now live among the Humans on Earth. The Humans have developed the ability to see our wings, wings the purest colour of white- that is, until we sin. The moment an Angel sins, their wings turn darker and darker gray, until they're pure black. This is our world now.