What Causes Painless Dent On The Forehead Of A Toddler?

My 22 month old fell and hit his head on a dresser corner. There was a half inch dent on his head (forehead), now there is a very small irregular lump under the skin that doesn t seem to bother him. This was last week and there has been no change to his demeanor and behavior. I expected it to go away and it isn t.