The mystery of the dark rainbow

Many years ago I saw a "dark rainbow". A perfect bow,  a dark curved band in the air, the same width and same shape as a normal rainbow. It was appearing in the south, where the dim sun also appeared, around 11:00 am. Later that day, my social network confirmed that others had seen this mysterious phenomenon as well. Mysterious indeed; nobody knew what it was or what could be the physical explanation! I contacted several meteorologists by email, but I never received an answer. All those years, none of the meteorological books that came in my hands provided a even the smallest clue... What could it have been?!!

More recently I saw a few times a similar phenomena with dark stripes instead of bows, which might explain the dark bow that I have seen. The last time, I finally managed to take a photo, trough a front window of a car.

It has a mysterious appearance indeed. It is neither a cloud or a stripe of dark particles, I think. Most remarkably is that the dark extends to parts of the sky where there is seemingly only cloudless blue. What can this be?
I suspect that in all cases, it concerns vapor trails from aircrafts. If so, they are lit from the back so that they appear dark (like the small cloud in the photo). Or alternatively, vapor trails that have almost fainted, but still cast a concentrated shadow on the clouds below. Being a shadow from higher elevations projected on the clouds, sounds even more reasonable to me, than back-lit particles. Feel free to come up with more hypotheses!
But how does the bow come in? Well, such trails could be curved. Hypothetically, when a plane makes a u-turn, you may see from the right perspective a perfect bow. I think the first time I saw this phenomenon, I was lucky that a plane turned almost 180 degrees, to correct for a landing on a nearby airport, causing a perfect dark bow to appear!