My perspective is that when debating on any topic, the definition of what you are debating needs to be agreed upon prior to proceeding. In this case, an expert in my eyes would be someone who not only has and shows knowledge, skill and aptitude in a certain domain but instead has close to 100% of the knowledge, skills and aptitude known in that domain.
My friends definition for an expert is someone who can apply what they know and become successfully repeatedly and also know why they were successful.
As you can see our definitions vary for the term expert.
If you use my definition for the basis of the debate I don't think anyone is an expert in any domain. Everyone is more or less knowledge than someone else. If you use my friends definition of an expert then yes, experts do exist in all domains of life.
The term expert is subjective. It is true that one person may have more knowledge, skill, and aptitude than another but at what point do they become an expert ?
We can take the example of an IT professional completing a vendor certification. That IT professional would be regarded as an expert in his field because in large part everyone in his field agrees to that level of education being indicative of an expert.
I found a great post at http://blog.asmartbear.com/expert-distraction.html. It discussed the relevance of being an expert to having success. I thought I would include it here. The following is what I had to say.
My opinion is that to be successful you don't need to be an expert, based on any definition of an expert. For example, people who can replicate the successful processes created by others are successful without ever being experts themselves. Franchises are a great example. Franchiser's refine their processes and systems until they have repeated success with them. They then franchise their processes and systems. The franchisee's can be successful without ever being experts in those processes or systems.