I agree with @scta132 & @Tim; & also with the counters; it’s both.
Of course there is back pain caused by physical injuries that allopathic medicine cannot heal, like a broken spine; other physical injuries it can heal, or at least treat the symptoms; & weak abdominals, excess weight, bad posture can physically strain lower back muscles, so need to be corrected, etc.
Many back injuries, particularly soft tissue, do heal, & once healed we need to tell our CNS its ok now, to stop avoiding the area, stop compensating & go back to using all muscles equally (this can be reinforced with exercises instructed by a PT or other therapist).
A great deal of chronic back pain though is thought to be “psychosomatic”.
For some this word has negative connotations, but it’s really not.
Stress, for example, is acknowledged as “a killer”, with many physical symptoms.
It is 100% psychosomatic.
It is created by our mind, & does not otherwise exist.
It appears many chronic back pain issues are caused by stress.
My favourite book on this is The Divided Mind, by John Sarno (MD).
https://www.amazon.co.uk/Divided-Mind-Epidemic-Mindbody-Disorders/dp/0715637274
" John E. Sarno, M.D., is Professor of Clinical Rehabilitation Medicine, New York University School of Medicine, and attending physician at the Howard A. Rusk Institute of Rehabilitation Medicine, New York University Medical Center "
He started treating back pain that allopathic medicine either could not cure at all, or could only temporarily “cure”, as it would inevitably return. Eventually allopathic back experts would refer all their difficult or “incurable” cases to him.
He also treats schizophrenics that have different physical diseases & symptoms that appear for different personalities, & completely disappear when the personality changes.
I read it many years ago, & stopped viewing the word psychosomatic as negative.
I decided I do not get headaches (an easy start, as I think mine were tension related).
Not one, ever, since. If I feel anything starting (rarely), I firmly remind myself & it stops.
Dr Sarno has written several others, such as ‘Healing Back Pain: The Mind-Body Connection’.
https://www.amazon.co.uk/Healing-Back-Pain-Reissue-Connection/dp/153871261X
(update: which I just realised is the same book that @scta123 referenced)