There are people who experience pain or itching in a limb that has been amputated. So even though the experience is real it does not prove that the object of that experience is real.
Furthermore, there is a difference between proving that my back pain exists and proving that back pain itself exists. Just wrench the back of the unbeliever and you will gain an instant convert to the existence of back pain.
I mention this because there is a linguistic sleight-of-hand in the believer’s response to the atheist’s challenge:
Atheist: What evidence, solid scientific, universally proven evidence do you have that your God truly exists?
Believer: What evidence, solid scientific, universally proven evidence do you have that your back pain truly exists?
Simply drop the word “your” from each sentence and you will understand my point.