Ana Bonaca: Galactic bullet holes

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?

00.06.26 

What are star clusters?

00. 08.00 

Is the milky way just a big star cluster? No, because the Milky way is bound together with a dark matter halo. 

00.10.15

The Milky Way has more mass than its amount of stars should indicate. So what is the extra, “invisible” mass? This is Ana’s favorite solution to what the cosmic bullets can be …

01.01.04

So, is earth safe from cosmic bullets?