Black holes are irresistible sinks for matter and energy. They are so dense that not even light can escape from their gravitational clutches. Massive black holes (equal to millions or even billions of solar masses) develop during collisions between galaxies. More ordinary, stellar-mass-sized black holes form as remnants of the explosive deaths of stars, and are thought to contain not more than about twenty solar masses of material.

Continued here: http://www.cfa.harvard.edu/news/2012/su201202.html