I was on board a southwest flight from indianapolis 8-15-2017 traveling to tampa non stop. flight was from 2:15-4:25pm. the sighting happened maybe 30mins after take off no quite sure the time. i was sitting in the middle of plane on the right side (window seat) looking west . right behind the wing. the pilot mentioned before take off that we were going to expect turbulence most of the flight due to weather. i sometimes am nervous while flying, so most of the time i am always looking out the window. i have flown many times in my life and have never seen what i saw. usually i just see other air planes passing by. with large clouds in the sky i noticed a large (compared to other airplanes passing by this was larger than an airplane) black elliptical (oval) shape ( up right )in the sky possible 2,000 feet below our 39,000 feet in the air, distance wise i can only guess maybe a mile or two. i believe the object was moving towards us. no white clouds (gas? that normally follows a plane that high in the sky). also it seemed to level out instead of being in an upright oval. my view only lasted maybe 20 seconds due to clouds that were between the object and us. i looked back to see if i could still view the object but i no longer could see. it was very strange and sounds crazy.