What could the hard bump that does not hurt and move under arm of a 28 year old be?

Possible causes of a hard bump under the arm that doesn't hurt and moves in a 28-year-old could include:

- Lipoma: A lipoma is a benign, slow-growing tumor made of fat cells. They are usually soft and movable and can occur anywhere in the body, including under the arm.

- Swollen lymph nodes: Lymph nodes are small, bean-shaped organs that filter and store waste products and harmful substances. When they become swollen, they can feel like hard lumps under the skin. Swollen lymph nodes are often a sign of an infection or other medical condition.

- Muscle strain or injury: A muscle strain or injury can also cause a hard bump to form under the arm. This can happen due to overuse of the muscles or a direct injury to the area.

- Skin cyst: A skin cyst is a small, sac-like growth filled with fluid or other material. They can occur anywhere on the body, including under the arm.

- Hematoma: A hematoma is a collection of blood that has leaked out of a blood vessel and pooled under the skin. They can occur after an injury or trauma to the area.

- Cancer: In rare cases, a hard bump under the arm could be a sign of cancer. If the lump is growing or changing in size or shape, it's important to see a doctor for further evaluation.

It's important to see a doctor if the bump is growing or changing in size or shape, if it's causing pain or discomfort, or if you have any other concerning symptoms. The doctor can determine the cause of the bump and recommend treatment if necessary.