There is such a thing as a symptom pool. Some people who are having a difficult time instinctively acquire symptoms that are fashionable and signal their difficulty, but the symptoms are not related to the actual problem. The prototypical example for this, IIRC, is incidence of anorexia in some specific country (Hong Kong?). An anorexia awareness campaign caused that symptom to become fashionable and increased the apparent incidence much more than can be explained as an increased ability to observe the cases that were present before.
This applies to some but not all of the social dark matter being discussed in the OP. Like cosmological dark matter, there is room for debate about whether some instances of social dark matter are real or not.
In response to “The real problem is humanity’s lack of rationalist skills. We have bad epistemology, bad meta-ethics, and we don’t update our beliefs based on evidence.”:
Another missing rationalist skill is having some sensible way to decide who to trust. This is necessary because there isn’t time to be rational about all topics. At best you can dig at the truth of a few important issues and trust friends to give you accurate beliefs about the rest. This failure has many ramifications:
The SBF/FTX fiasco.
I quit LessWrong for some years in part because there were people there who were arguing in bad faith and the existing mechanisms to control my exposure to such people were ineffective.
Automata and professional trolls lie freely on social media with no effective means to stop them.
On a larger scale, bad decisions about who to trust lead to perpetuation of religion, bad decisionmaking around Covid, and many other beliefs held mostly by people who haven’t taken the time to attempt to be rational about them.