Well - quite a few of us have the impression that their onset is virus-related, in my case praps swine flu, and this possibility is also mentioned in studies, but far from being proven. Experiences of CoV and especially long-CoV show much more symptoms which may look like FM, so quite a few people hope that CoV-research will help us, too. One of the problems with this is that these symptoms are in common with most chronic pain illnesses, and these overlaps could be much more of a problem than a help.
The other way round brain fog has similarity with ADHD, which has made a study conclude that 45% of fibromites have ADHD, badly constructed, as you may have guessed. So these similarities unfortunately won't get the researchers and us anywhere.
Viral infection'd come closest to the theory that some sort of autoimmune inflammation or similar (praps influenced by ATP/mitochondria) - not detected by current normal inflammation-screening - may be a cause (in one set of people). Seeing studies that claim to know the answer still contradict each other, sometimes even themselves ;-), I prefer to follow developments closely, but 'closely' only in the sense of with due care and reservation. Or rather I favour studies which themselves carefully consider/discuss all of the 3 or 4 theories that are being tentatively thrown around at the moment. I've been sort of following medical progress since the 80s (or then compared to now) and am dubious that any research will help us sooner than 2040. Just read the latest I think well-constructed review (Jan 2021) on our treatments, stating that they are generally not proven*. I have far more trust in people getting to know their bodies, comparing with others here in forums, and finding their own answers as to what can help.
* "Association of Therapies With Reduced Pain and Improved Quality of Life in Patients With Fibromyalgia - A Systematic Review and Meta-analysis, 2021 (Oct 2020): "This systematic review and meta-analysis suggests that most of the currently available therapies for the management of fibromyalgia are not supported by high-quality evidence. Some therapies may reduce pain and improve QOL in the short to medium term, although the effect size of the associations might not be clinically important to patients." and "Findings: In this systematic review, the effectiveness of most therapies for fibromyalgia was not supported. Strong evidence supported only cognitive behavioral therapy for pain, as well as antidepressants and central nervous system depressants for pain and quality of life, but these associations were small."
