I was on a flight leaving from london around 2pm and landing in tel aviv around 9pm. while we were over austria i happened to look outside the window and i saw what seemed to be an abject flying the opposite direction. at first i was like oh a bird. then i saw how fast it was moving and that there were no wings flapping. also giving how high we were in the sky. i look back at it. the object appeared to be flying at two times the speed of the airplane i'm in and took some crazy turns at crazy speeds but it seemed to be going on a certain path at the same time. before i knew it, it was already gone. the object appeared as sort of a boomerang shape. all black with what seemed to have lights surrounding the outline of the boomerang. i immediately check the flight map to see where we were and it was a location between salzburg and leoben, austria. the object appeared very low also, it seemed as if it was riding the clouds. definitely not at same altitude as us. probably also a couple miles away. the size seemed like it was probably a big craft of some sort. hard to tell. i was so shocked by seeing it because i never thought something like this would happen. it didn't even come to my realization until afterwards to record the event.