It could be caused by the shape of his hairline, a slightly slapdash barber or simply a trick of the light.
But bloggers in America are convinced the ‘scar’ on Barack Obama’s head has a more sinister explanation.
Conspiracy theorists claim the line, running up over the president’s crown from behind his right ear, proves he has secretly had brain surgery.
President Obama meeting comedian George Lopez as the First Lady looks on. Some observers believe this image shows a distinctive scar which looks like those left after brain surgery
Left: Obama's head from a different angle. Right: An enhanced image which purports to show the scar running from the top of the President's head to behind his right ear
They suggest it could be anything from a routine procedure to the work of extra-terrestrials, and have posted enhanced images of his head on Right-wing blogs as ‘evidence’.
Commentator Ben Hart wrote on Conservative discussion forum EscapeTyranny.com: ‘Was it a brain operation? Has it affected his thinking? Is he really an alien from another planet?’
The White House dismissed the theories as ‘ridiculous’.
The Escape The Tyranny website strongly questions Obama's head scars as well as why no one has seen his birth certificate