While studying the stellar stream GD-1, Harvard astronomer Ana Bonaca made a literally enormous discovery: Cosmic "bullet holes" in our galaxy, several light years across. Something with around a million times the mass of our sun has punched enormous holes in our galaxy. What is the "bullet" here? Is it a supermassive black hole, of the kind we only see in the center of a galaxy? Or is it a cluster of stars? Or is it something even stranger, a "bullet" made of dark matter?