So, here's the thing about black holes. They tend to be either super-massive vortexes in the center of galaxies that are, as Carl would put it, millyuns or billyuns of times the mass of our Sunn - or a remnant stellar black hole of between 3-20 solar masses.

We think we know how the smaller ones form. Supergiant star goes up the periodic table, fusing heavier and heavier elements for fuel, until its temperature and hence its outward pressure drop below key gravitic thresholds. Result: implosion, accompanied by rebound and a cataclysmic supernova explosion that blows off most of its mass. If last-stage atomic forces can hold the tiny remnant up after everything is collapsed into neutrons or quark "degenerate matter," you get a neutron star/quark star, where one teaspoonful would weigh about as much as an earth mountain. If it's a more massive remnant, however, it will continue collapsing in size, without losing mass. What's left makes such a big dent (hole? hard to say) in space-time, that even light ends up circling the drain and unable to escape if it passes within the thing's "event horizon".

What we don't know, is how the super-massive black holes form. The most popular current theory is merger: black holes combined. OK. But if that's so, there should be more of a size continuum. We should see mid-size black holes that are larger than we could expect from a single star's collapse: somewhere between 100 and several thousand solar masses.

The first strongly-confirmed example may have come in 2004, when the Hubble Space Telescope found one at the center of the giant G1 globular cluster near Andromeda galaxy. These medium-size black holes have been theoretically linked to some of the massive x-ray bursts we pick up now and then, but they aren't the only thing that could cause them. That's why the recent HLX-1 discovery using the European Space Agency's XMM-Newton X-ray space telescope may have just added a 2nd medium black hole to the catalog - and an important piece to the puzzle.