Ganglion Cyst: A common, benign lump that forms on the wrist, often on the dorsal aspect. It is typically firm and rubbery, which matches the patient's symptoms. The lack of trauma and the description of the mass make this a likely diagnosis.

Tenosynovitis: Inflammation of the tendon sheath, which can cause pain and swelling on the dorsal aspect of the wrist. Although it might not typically present as a firm, rubbery mass, it's a possible cause of wrist pain and swelling without trauma.
Rheumatoid Nodule: Associated with rheumatoid arthritis, these nodules can form over bony prominences, including the wrists. They are firm and can be mistaken for other types of masses.
Lipoma: A benign tumor composed of fat tissue, which can occur almost anywhere, including the wrist. It might feel firm and rubbery, depending on its composition and location.

Osteosarcoma or Other Bone Tumors: Although rare, bone tumors can present as a firm mass and must be considered to avoid delayed diagnosis of a potentially life-threatening condition.
Infectious Process (e.g., Abscess or Osteomyelitis): An infection in the wrist area could cause pain, swelling, and possibly a palpable mass. It's crucial to consider infectious causes, especially if there are signs of systemic infection or if the patient is immunocompromised.
Gout or Pseudogout: These conditions can cause sudden, severe wrist pain and swelling, often without a history of trauma. While they might not typically present as a firm, rubbery mass, they can cause significant joint inflammation.

Synovial Sarcoma: A rare type of cancer that usually occurs near the joints of the arm, neck, or leg. It can present as a slow-growing mass.
Giant Cell Tumor of the Tendon Sheath: A rare, benign tumor that can occur on the wrist, among other locations. It might present as a firm nodule.
Neurofibroma: A benign nerve sheath tumor that can occur anywhere, including the wrist. It might feel firm or rubbery, depending on its location and size.