Here is a bit more on the thinking that led this researcher to “the slingshot conclusion” if you will:

“Looking around where the supernovas exploded, there’s nothing there – no trace of star formation, no clusters of old stars, there’s nothing nearby,” Foley said. “So I knew that these things were starting somewhere else and moving long distances before they die.”

Examining the locations and kinematics of the supernovas, he was able to determine that the stars that exploded had been kicked out of their galaxies at very high speeds, millions of years before they exploded.

To understand how the supernovas got so far from their galaxies – up to half a million light years away – moving at such high speeds, he looked at the galaxies that had produced the stars before ejecting them.

“Whatever put the star system in the state where it’s about to explode is related to the center of the galaxy it came from,” Foley said.

What specifically however would cause a galaxy to “kick out” a star like that.  Dark mattter?  A case of black hole indigestion 😉 or something else?

thanks to phys.org for the great info