I had an x ray at the dentist which showed a dark spot next to my very back tooth..what could this be?

Where is it ?: Is the dark spot in the bone or in the tooth? Big difference between the two. Ask your dentist to clarify.

Xray: ask your dentist to review the X-ray with you and explain what the dark spot is. It is not possible to answer your question without seeing the X-ray.

Bony defect/loss: If it was not in the tooth but in the bone behind the tooth then it may be a bony defect, bone loss from periodontal disease, an abscess which has caused bone loss, or a cyst or granuloma or other defect. Best to further investigate and see an Oral Surgeon if needed. Many possible pathologies. Good Luck.

Dental abscess: A dental abscess will show up as a black spot on the x-ray. This is the source of infection and should be treated accordingly.

Dark spot: Dark spot usually means absence dense tissue. Teeth, bone, metallic fillings, etc usually appear white. Cavities, bone or tooth loss, etc. usually appears dark. As we are not prove to your x-ray we can't give you a definitive answer. Return to your Dentist. Ask him/her to advise as to the nature of the dark spot and the appropriate treatments that are available.