A hard bump on the lower back could be a relatively large subcutaneous lipoma. This is because smaller lipomas are softer and have clear borders with fair mobility. But if it is a hard bump, it could be a huge lipoma or a sebaceous cyst, which is not easy to determine. As for what causes it, there may be trauma, localized fatty deposits, the cause of which is not very clear, or other lumps, or localized tumor-occupying disease or muscle herniation. If there is no discomfort, just continue to observe, or go to the hospital to improve the ultrasound examination for preliminary determination. If there is no particular problem, surgery can be performed or observation can be continued, but avoid repeated rubbing and massage, otherwise it may lead to local lesions. If surgery is performed, pathological examination should be performed after surgery, which is the gold standard for differential diagnosis.