Suggest Treatment For Dark Bruise On Thigh

I'm 15, about 155cm and weight approximately 50kg. My medical history is not relevant to this as I've had no such issue that can be related to this. I have this very dark bruise on the back of my thigh, very close to my knee. There is no pain whatsoever, although it appears to be a very dense purple and is quite swollen. I'm not quite sure what it is, do you possibly have an answer?

Thank you in advance.