Thank you for your question about your dachshund. I am glad to hear the lump does not seem to bother him.
There are several possible causes for a lump under the skin that is not painful. It could be a cyst, an abscess (these usually become painful), or a tumor. Tumors can be benign or malignant. One common type of benign tumor is called a lipoma. It is a build up of fat cells. They can enlarge but do not spread to other areas of the body. Malignant tumors can spread to other parts of the body.
The only way to diagnose what kind of lump it is would be to have your veterinarian take a needle/syringe to collect a small sample from the lump and look at it under the microscope. If the lump is a benign cyst or lipoma, they may tell you to monitor the size. If there are suspicious cells, they may recommend having the lump removed.
Even benign lumps may need removal if they continue to enlarge. For example, my own dog has a lipoma (fatty lump) in his neck area as well. It is much larger than your dog's. Because it is in an area near important blood vessels, I plan to have it removed before it gets larger. If the same lump was on his abdomen, I would probably just watch it.
Please let me know if you have any other questions or concerns. I am here to help.