This is a great and spooky image!
From a few recent APODs, I have wandered into an area that is new to me. The question, in general, is how much of the in-principle-observable universe of EMR-emitting objects are we able to see (fairly clearly), and how much of it are we blocked from seeing, by the simplest of reasons, of having clouds intervening that are effectively opaque, or other objects which are opaque. Here I am saying opaque in the very empirical sense that there is a star or galaxy in a given direction, whose light has had time to reach us, but we don't see it in our telescopes because of what lies in between us and it. (I'm not talking about things we can get around, such as the Earth or Moon, or things that intervene occasionally, such as Jupiter. I'm talking about things that are far enough and sufficiently stable that we are truly at their mercy. We would not necessarily know they are there, but we also fail to see anything beyond them.)
A nice example from today's APOD is the excerpt at the right. It is clear that there are almost no stars shining through the dark brown patches to us, at least not visible in the image. Therefore, a survey of this region would be impoverished in its star count. I think this is a good example, because it is so opaque as to clearly show the point. But that also makes it a bad example in terms of subtlety. The most problematic case would be clouds that are subtle enough that we do not note them.
Chris pointed out that if there is too much of this going on, we would be able to tell of its occurrence because such clouds would glow in IR. Although I find this statement hard to contest if one imagines a simple curtain-like shield over a lot of bright objects, I'm not prepared to agree with that if one considers all the subtlety of arrangements that are possible in our huge, 3D universe, that it is indeed impossible that there is a significant amount of blockage out there that really does block light and that we cannot detect, or at least have not yet detected.
I don't know where I'm going with this, it just seems that I'd like to get a better handle on whether or not there could be this source of uncertainty. Also, whether or not one could make estimates of the issue and how one could hope to do so. If there is sufficient "hidden normal matter", then it would affect estimates of the amount of truly dark matter, I would think.
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