Rainbow of Wavelengths video.) The question was how common are dark plasmas? When we look at the Sun in EUV wavelengths it is actually quite common to see both bright and dark regions. The triptych shows AIA 193 Å, HMI magnetic field, and AIA 304 Å, all from 1800-1830 UTC today. I identified several dark regions, a coronal hole and three filaments, which will morph into prominences when they rotate onto the edge of the Sun.
Coronal holes are dark because there is very little material in that part of the corona. No material means no glow. As solar maximum starts to wind down we will probably see many more coronal holes on the Sun. The ones that form at the poles are particularly neat, as they can last for several years as little caps.
Filaments are dark for the opposite reason. There is too much material and the light from anything beyond the filament is absorbed by the filament. More or less like holding your fingers in front of your face to keep bright light out of your eyes. It is also cooler material and doesn't glow as brightly. Filaments like to form along the lines where oppositely pointed magnetic fields meet. That is easy to see in the filaments on the right (one with an arrow, the other just the letter F.) Look for the grey area between white and dark magnetic fields in the magnetogram at the same place as the filaments in the EUV images. This line is called the "Polarity Inversion Line". Can you see the Polarity Inversion Line in the filament on the left? It will probably get easier to see as it rotates towards the center of the disk.
The coronal hole has only outwardly directed magnetic field (white in the magnetogram). The field is not as strong as near the active regions and it is usually open, or goes far out into space.
The Rainbow of Wavelengths video shows how the many wavelengths of light that SDO measures allow to see the Sun in many different ways. Bright or dark, our job is to understand all of them.
Enjoy the solstice and keep watching the Sun!