I think to qualify as an expert on dark matter you should at least be familiar with General Relativity, the standard model of cosmology, gauge theory (and in particular extensions of the standard model of particle physics), and have a good overview of the literature on dark matter experiments. Add in a novel contribution to the literature, and you have done the bulk of the work required to gain a Ph.D.
It is very difficult for a poser to fool actual experts. It is a lot easier for a poser to fool non-experts. Also it's often more lucrative.
Experts may disagree with one another without either party risking be considered non-expert by her or his peers.
Experts, also, having a decent overview of the literature (which includes observational and experimental results) will generally have a decent understanding of what they don't know. It is very rare for a poser to admit what he or she does not know. It's also fairly common for non-experts, who are not experts in some other discipline, to have no idea the amount of knowledge experts have been exposed to during the course of developing their expertise.
> No doubt they are very smart
Really, it's more that actual experts are excpetionally well-read in their narrow little disciplines, and along the way have lots of practice writing as well. The key to expertise in theoretical physics is actually reading theory (and the results of tests thereof). A lot. Writing enough will probably result in a dissertation. And, perhaps, getting a paper published, writing a book or a chapter of a textbook, and so on.
The other thing you have right is that experts can be wrong. But being wrong usually offers up an opportunity for more writing that other experts are likely to get around to reading. So being wrong can increase expertise, as one learns from others' mistakes and how they were caught, and as one thinks about how to avoid similar mistakes in the future (which offers up another opportunity for writing... and so on and so forth).
> I guess it gives me hope I may one day be an expert at something.
Noone becomes an expert without effort.