Noun. Expert skill or knowledge in a particular field.
You are the best in the world on the subject of your thesis.
You did the research. You wrote the thesis. Thousand and thousands of hours of work.
It will take people with expertise to read your thesis and get something from it.
No-one else will have the expertise that you do. That might not be enough to help you feel completely confident for your viva, but it’s a good starting point.