In the 2011 Sixty Symbols video Spy Satellites (from Deep Sky Videos) after about
01:44 amateur astrophotographer Nik Szymanek shows an image of a cluster of stars and I think I see what looks like a dark donut shape (dust?) to the right of the center of brightness, as shown in the screenshots below.
The vertical streak is a satellite trail, the topic of the video but not related to the present question.
Question: Can someone recognize what object this is and explain what is thought to cause the dark donut?
Screenshots from video, click for full size:
Cropped and shamelessly processed to enhance the dark donut's visibility: