Some may not know this but SME = Subject Matter Expert!
There are many aspects to being a subject matter expert. First rule being you do not know everything. You may be a mechanic who knows a lot about a certain brand or type of vehicle, but, are you an automotive mechanic or aircraft mechanic or marine mechanic? Part of being a subject matter expert is narrowing down to your particular area of expertise. You are the one that knows what you know, and you are the one that can narrow that down to one or more specific areas of your field.
One example of not knowing what your area is would be saying you are an internet expert, you may know a lot about the internet and the way it works, but you are not an internet expert. You may know a lot about internet security, but you do not know it all, guaranteed. You may know SEO (Search Engine Optimization) but again, you do not know it all.
As much as you may think you know, you need to keep learning more, evolve and develop your knowledge and experience. You can learn from many sources from online videos to documentaries, to ebooks and much more. The key is to keep up to date, and constantly be learning. Part of learning may involve reading things you do not believe in especially online, but, read the information out there. You can correct not correct information by writing your more accurate articles and posting these where you can. Correcting wrong information can help establish yourself as more of a subject matter expert.
With SEO efforts, the more you project, through your entire online presence as a subject matter expert, the more your information can be considered authoritative. You need your readers to believe in your presented information, they have to be able to understand your points regarding your area of expertise. You do not need to worry about what you know in particular, just present yourself as a authority on your chosen subject. This can be as simple as say knitting, if you know a lot and are great at knitting, then establish yourself as the go to online authority. Share your expertise with others.