I am concerned about this dent in the middle of my forehead, that seems to be getting progressively worse (Photo)

Although this condition is more specific for pediatric population, one should consider possibility of linear scleroderma (en coup de sabre) deformity which is progressive and may be associated with neurologic disorder which develops later one. A consultation with a dermatologist and plastic surgeon should be your first step. Treatment may consist of fat and nano-fat grafting which may need to be repeated due to progressive nature of disease. Good luck.

The furrowing on your forehead may be as a result of excessive muscular tension in the area. The action of several muscles around the glabella could be the cause as they become enlarged. As the midline area is devoid of muscle to varying degrees it may create a furrow as the edges of the enlarged muscle become defined. Botulinum Toxin injections are reversible and may help to reduce the furrowing and reduce muscle definition.

You appear to have enlarged and hyperactive corrugator muscles which can create fullness under the eyebrows and a relative depression between the eyebrows. I would try Botox to the corrugator muscles (under and between the eyebrows) to relax this musculature and see how this effects the forehead contour. Fillers could be added to further improve the contour of the area if Botox does not give you the desired result but I suspect you will see a big improvement with Botox.