Led by George Daley of Children's Hospital in Boston, the researchers began their process by culturing mouse embryonic stem cells to form globular clusters called embryoid bodies.

NPR: Making Sperm, No Men Necessary

"Some theories say that small black holes in globular clusters should sink down to the center and form a medium-sized one, but our discovery suggests this isn't true, " said Daniel Stern, an astrophysicist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

MSN: Existence of middleweight black holes nearly ruled out