dark shaped floating object surrounded by clouds.

This photos are taken by me in a plane from Surabaya to Jakarta (Indonesia - South East Asia).
I look from the plane window and saw this "thing" just floating.  At first I thought it was a dark cloud or just a stain in the window.
Then I realized something was odd because it looks like the object was moving in the opposite direction against the plane (I still have no idea whether it's just standing there or it's actually moving).
In my opinion that is definitely not part of a mountain because it was floating in the air nor a cloud because the color of surrounding clouds are bright yet the color of the object is dark.
Supported by the fact that the shape of the object is quite slim and thin, which doesn't resembles the shape of mountains or clouds because it's way too slim.