There is no scientific or substantial evidence that angels exist. Although thousands of people purport to see angels each year, scientists believe these encounters with angels are a survival mechanism of the brain and not winged guardians.
Scientists believe that in moments of mortal distress, the brain experiences both extreme terror and a deep peace. This feeling is meant to help people get through the situation. In some cases this, feeling becomes personified into the image of a being. In addition to the image of a being, people also report voices speaking to them. These people then interpret those experiences as a visit from their guardian angel.