It seems that most of the DNA evidence that I have at present is like trying to identify very faint stars on a dark night. You know that they are there, but you cannot be certain exactly what you are seeing. Is it a star, or a figment of your imagination? We want to discover new connections with hitherto unknown relatives. The evidence is fuzzy.
Ancestry has identified (at the time of writing, January 2023) 427 fourth cousins or closer. There are over 10,000 matches in total. But they are all distant stars. Even the fourth highest on my list shares only 44cM of DNA with me. By working from both ends, my fellow researcher and I have managed to identify our common ancestor. Dear old George lived from 1726 to 1783. Crucially, his entire life was spent in Sussex, where records are plentiful. We are, according to our respective trees, half fifth cousins once removed. I was also helped by the fact that my distant cousin has a variant of the name of one of my great grandfathers.
Research published by the International Society of Genetic Genealogy (ISOGG) shows that the probability of detecting no discernible DNA between fourth cousins is 30.7%. For fifth cousins once removed, that probability is over 80%. This means that my fellow researcher and I were quite lucky that there was so much DNA with which to work. https://isogg.org/wiki/Cousin_statistics
According to the chart available on Family Search, https://www.familysearch.org/en/blog/centimorgan-chart-understanding-dna half third cousins once removed may share anything between 165 cM of DNA and zero. My (admittedly unsubstantiated) belief that I am related, in some fashion, to other branches of Roscommon based Stanley families with whom I share no DNA is based on this fact.
I am left with trying to determine the relationship with dozens of people with whom I do share frustratingly small amounts of DNA. Given the maths involved, if there is only a 20% chance of finding common DNA, then I need three such people to push the odds to 50% of finding a match with a specific distant ancestor. Perhaps it is a good thing that there are so many matches on my list. Unfortunately, a great many of those people have no family trees. I continue to look for faint stars.