dslr – What is this weird dark spot on my viewfinder that looks like a microorganism?

weird dark spot in viewfinder
I recently bought a used Canon 1100D, and I didn’t notice this… defect? dirt? chip? crack? dividing organism?! — I don’t know what to call it — in the viewfinder until I brought it home.

It’s actually quite hard to notice unless you hold the camera a little bit away from your eye and try to focus directly on it. In the photo above, I did my best to bring the… thing… into focus with my other camera. I had max out the affected camera’s dioptric adjustment knob to get the image above.

The image below best represents what the “thing” looks like when you look through the viewfinder yourself — it’s the dark splotch on the upper-most AF point.

foggy dark spot from viewfinder POV

The “thing” does not show up on images. And it does not seem to be on the mirror or focusing screen (see image below). When I tilt the camera slightly from side to side or up and down while looking through the viewfinder, there is a parallax effect, the “thing” appears to be between the viewfinder lens and the focusing screen (the AF points move a greater distance than the AF points when tilting the camera).

mirror and focusing screen

What could this be?