I was looking out the left side of the plane, and seen "this object" which was approximately 150 to 200 feet off our left wing. It appeared to be 10 to 15 feet in diameter. It was slightly higher than the tip of the wing from my perspective. We were at cruising altitude of 35, 000 feet.I first noticed this object was "keeping pace" off our left wing. It was in a fixed position "off our left wing" for 30 seconds before I pulled out my smartphone camera. The plane speed was approximately 485 mph. So, this object speed had to be 485 mph for the first 30 seconds I first saw it.I immediately thought "I'm witnessing a UFO". I have also seen one when I was a kid (about 35 years ago). The object started to change speed (it slowed down) as soon as I started to video record it. The video recording starts with the object "slowly" slowing speed, and starting to fall behind the rear-left of the plane.I started to truly believe in "intelligent life existing on other planets" when I seen my first UFO as a kid. This incident only "confirms" my life-long held belief in the aforementioned. I lost "visually" sight of the object shortly after I started to video record it. 75 % of this video recording was my "hoping" that my smartphone was able to capture this thing. I was correct! The video recording shows the object "without" zoom, and depth perception.