Black holes are the universe's trap doors leading into an abyss. They normally come in two sizes. The first type are stars that are massive enough that when they burn out they collapse down to a singularity that's equivalent to five to ten times our sun's mass crushed into something smaller than the period at the end of this sentence.

The second known type are monster black holes that are millions or even billions of times our sun's mass. They dwell in the dark hearts of galaxies, gobbling up any stars that pass nearby. They must be intimately tied to the early days of galaxy formation.

What's elusive are so-called intermediate-mass black holes weighing hundreds or a few thousand solar masses. Are they a "missing link" in black hole evolution?

One of the best candidates is lurking in the center of the huge globular star cluster Omega Centauri. Because black holes cannot be directly observed, astronomers must use "stellar forensics" by measuring how a black hole's gravitational pull affects the motions of nearby stars. Stellar velocities can be used to estimate the black hole's mass. The faster a star moves, the more powerful the tug from the black hole.

This is a meticulous process that only the Hubble Space Telescope can do for something as far away as Omega Centauri. Astronomers used more than 500 images from Hubble — spanning two decades of observations — to follow the motion of seven fast moving stars in the innermost region of the star cluster.

The black hole in Omega Centauri is estimated to be 8,200 times that of our sun. If confirmed, it is closer to Earth than the 4.3-million-solar-mass black hole in the center of the Milky Way. That black hole is 26,000 light-years away, while Omega Centauri is 17,700 light-years from us.