Inverted Y-shaped object seen from commercial aircraft at 28K ft

September 02, 2021, 14:15 Eastern Time, United Airlines Flight # UA3412, window seat D (near aircraft’s right-hand, just forward of wing). We were somewhere near the borders of North Carolina, Kentucky, and West Virginia. Flightradar24 mobile app says we were at an altitude of 28,000 feet. Weather was fairly clear (some clouds).
 For approximately 10 seconds, I saw one, white, inverted (upside-down) “Y” go past our right wingtip. In the distance was another white aircraft (presumably civilian).
 I served in the Air Force for 6 years as a 2A3X2 ( F-16 Avionics Technician on the flight line), and experienced many aircraft in the US defense inventory. Additionally, I worked for a civilian aircraft manufacturer for 10 years. This was unlike any aircraft I’ve ever seen. As a skeptic, I have been researching what it could’ve been for about a month now, and I don’t have an explanation or a viable theory. Maybe an experimental aircraft in a vertical stall??