Boomerang shaped object seen in daylight at Great Falls, Montana.I was at my granddaughter’s softball game and happened to glance up at a dark cloud above and slightly west of me. I saw an object which at first I thought was a high flying eagle but I saw immediately that the object was stationary and not changing shape. It was dark colored and I would describe the shape as a boomerang with squared off tips.I was about to draw my wife and daughter’s attention to the object when it started to move and appeared to dive. After a second or so I realized it was not diving but moving away to the west at an incredible rate of acceleration. It stayed below the cloud level as I could see it until it disappeared behind the golf course tree line to the west of the ballpark. All of this occurred in less then ten seconds. The object did appear to change shape but I am not sure if it really did or just appeared to as it moved away at a high rate of speed.I have been around civilian and military aircraft all of my life. My father was a flight instructor and I spent 4 years in the USAF and 28 years in the Montana Air National Guard as a civilian technician. I am very familiar with military aircraft and I can tell you for sure that this was not a military aircraft. I have never seen anything, with the possible exception of an air to air missile, accelerate at the rate this object did. If I was going to guess at the rate of acceleration I would say this object went from stationary to well over mach 1 in about 3 seconds. I would estimate that from the time the object started to move until it was out of my sight was less then 5 seconds. The object made absolutely no noise and there was no sonic boom. The Air Guard here has F-15s and there were none flying in the area at the time of the sighting nor did I hear any after the sighting. I continued to scan the sky for about the next 15 minutes but did not see the object agai! n.