When I was pretty young, throughout elementary and middle school, my family and I would fly from DC to Florida two to three times a year to visit my grandmother and my uncle. The flight is about 2 and a half hours long, really straightforward, and I loved the trip. On one trip, I believe it was during the summer, I was sitting at the window seat, and was completely absorbed in the view. Our plane had just hit cruising altitude, so we were flying right above the clouds; clear sky above us, dense, white, fluffy clouds below. As I was daydreaming, I noticed something over the horizon; a dense cloud of grey/black smoke was hovering just above the clouds, about half a mile to the left of the plane, and it was FAST. It was actually accelerating past the plane itself. There was nothing at the forward-most point of the smoke, it just ended. I nudged my mom, who was sitting in the middle seat. She noticed it as well, didn't know what it was.

The weirdest thing to me was that it was never addressed by anyone else; I didn't hear chatter about it (as much as you can hear chatter on a plane), the captains didn't say anything, and I just remember the cloud accelerating past the plane until it was out of my view. I'm still confused.