Walking the dog, I look up to the skies as I use to do from time to time.By chance, I discover a dark shape moving in the sky. No lights, almost tenuous. I thought: What a large bird.I kept on watching to see if I could tell which kind of bird it was, as huge birds are not usual around here. But almost instantaneously I thought, that is not a bird, must be a delta wing? No sound, no lights...Then I started to see it more clearly its shape against the stars. That is not a delta wing either. It was crossing the sky just too fast, and it seemed huge and very high. Besides it was doing such a perfect straight line.All along the watching, but specially when it was at the closest position over me, I noticed that there was some sort of "cloud" that seemed to emanate from the solid shape itself, but it was not leaving a trail, it was like vapour all around the structure.In a night with no clouds, one single strange pentagonal cloud was crossing the sky at a speed too high for being a cloud (besides there was no wind either), and indeed, it was not a cloud, a solid shape was perfectly distinguishable.I followed its movement with the eyes, and eventually blocking the street lights with my hand as it was farther and harder to see, until eventually it disappeared in the distance. Not yet a dot as apparent size, but not able to tell anymore where it was with the naked eye.Extended information:-Location: Center of the city of Madrid, Spain.-Very clear night full of stars. Sky was not yet dark, but rather deep blue.-The object crossed the sky from South East to North West.-The object seemed to pass at a very high altitude.-The object was dark grey and dull, almost absorbing light.-The object was hard to discern, but it was occluding the stars.-The object had a solid pentagonal shape, that seemed surrounded all around by a dark grey fog, as some sort of cloaking.-The object had an apparent size of a full moon.-The object speed was too fast for being a balloon, or a real delta wing. In fact, too fast for being a jet plane. It crossed the full sky in around 10 seconds.