What is the indent above my belly button? (Photo)

Your belly button may have been stretched  to accommodate a large pregnant uterus and you now may have an umbilical hernia.  You should be examined to see if this is the case and you may consider a hernia repair. This would be a surgical procedure.  There are no non-surgical treatments for a hernia.

As you know it is not possible to determine the specific cause of the indentation without an in-person exam but it appears to be a muscle separation. Your belly button is located a bit high, which is an ideal situation for a mini-abdominoplasty (mini tummy tuck) with an umbilical float. That would allow for repairing the muscle separation without a scar around the belly button. If it is a muscle separation from pregnancy, it cannot be corrected with exercise.

The muscle tightening sutures run from the sternum to the pubic bone, skipping over the belly button. The muscle repair sutures may have ended above the indented area, allowing the muscle to spread in that area. The problem can likely be corrected with a small incision at the navel to access the area and place sutures to re-approximate the muscle. If that is indeed the problem, a smooth contour should result.

Those look like stretch marks to me. I believe they are indented because the skin is very thin and there is no muscle attached behind it in that area. There are some radiofrequency "lasers" that may work to tighten the skin, but I have been disappointed by the results. Given where your stretch marks are, I think they could be cut out surgically and sutured back together carefully, leaving a very slight scar above (or right on the upper border) of your belly button.