Dr. Sean Kirkpatrick, who leads the Pentagon's "All-domain Anomaly Resolution Office," talks about efforts to determine the truth behind "unidentified aerial phenomena" sightings around the planet. WATCH NIGHTLINE EPISODES: abc.go.com/shows/nightline ALSO AVAILABLE ON HULU: hulu.tv/2wSmSrZ #nightline #uap #ufo #pentagon #anomaly #phenomenon #investigation #abcnews