My doctor said, "You have 1 year to live."
I said, "You wanna bet?"
Bam, a gunshot!
Listen up, dummy. You're so dense, light bends around you. The joke is dark because the dude's doctor gives him a year to live, and he responds with "wanna bet" and then shoots the doctor, implying he's betting his life he won't die in a year because he's cutting his life short now by committing murder. Get it now, Einstein?