Lower right back teeth ache but my dentist can not find anything wrong. any thoughts?

Second opinion : If your tooth is really hurting, get a second dental opinion. Could be your wisdom teeth.

Maybe cracked: Your tooth may be cracked. It's hard to see that on an x-ray, and even sometimes with visual inspection. If the pain doesn't stop, ask your dentist to check for a cracked tooth with a light. When shined on a cracked tooth, we can see where the light shining through gets blocked.

Endodontist: Endodontists are experts in diagnosing toothaches. Many cracks cannot be seen either visually or on x- ray. Endodontists who work microscopically can find and diagnose much better.