I'vehad this bruise on my knee for about three weeks. i have a ping pong sized hard bump on it. it is black and blue with a light center? what is it?

Hematoma: This is a benign blood clot due to some type of trauma. It will not be any danger to you. As it resolves the center can be lighter. You can hasten its resolution with application of external heat and hot packs. This will help the body remove the clotted blood. Your physician can confirm the diagnoses and see if other issues should be considered. Sometimes it can be aspirated.