"Every Sunday, when we pray the Creed, we profess our faith in things visible and invisible," said Fr. Gary Selin, who is a Formation Advisor, as well as an Assistant Professor, at St. John Vianney Theological Seminary in Denver. "The invisible refers primarily to the angels, that God created both material things and the spirit world..."
See also this Denver Catholic article from 2012, "Angels: the invisible heroes among us," in which Fr. Selin is interviewed.