Ironically, black holes aren't too hard to find. They're bright.
Here's a bunch of 'em:
Every dot of light in that image is a black hole, hundreds of millions and even billions of light years away. Before you say, "Say wha?", this will take some explanation.
The black holes themselves are black (duh). But as matter falls into them, it can settle into a disk, called an accretion disk. If you remember your college physics -- you did take college physics, right? -- as something falls into a black hole, it acquires a huge amount of kinetic energy (for you pedants, it actually converts gravitational potential energy to kinetic energy). Think of it this way: when you hold a rock up over the ground, it has potential energy-- the potential to move due to gravity. When you let go, that potential energy becomes kinetic energy-- the energy of motion. If you don't think it has energy, then let it hit your toe. The kinetic energy will be converted to a loud crunching sound, and you will potentially have to go to the hospital.
So matter falling into gravity can gather energy, and matter falling into a black hole can get a lot of energy. This is converted to motion and heat, and as the matter piles up into the accretion disk it gets terribly, terribly hot: as hot as millions of degrees. There are also associated magnetic fields and other forces which can make the matter in the disk light up, getting it extremely bright. The bigger the black hole, the brighter this disk can get.
Astronomers think that in the center of every big galaxy there is black hole with millions or even billions of times the Sun's mass. Guess how bright they can get?
Answer: pretty damn bright. In fact, as long as they are actively feeding, black holes like this are the brightest sustained objects it the Universe. We call them "active galaxies". They're so bright they they can be spotted when they are billions of light years away. And hey, didn't I say that the spots in the image above were at that distance?
Yes, good! You're paying attention. The image at the top of this entry is from the orbiting Chandra X-ray Observatory -- it's only one part of a bigger image revealing 1300 black holes at the centers of galaxies.
When matter gets heated to millions of degrees, it gives off X-rays, so Chandra is a great telescope to spot black holes, especially the supermassive monsters in the centers of galaxies. We're still trying to figure out just how many galaxies are active, and how many are quiescent like the Milky Way (our central black hole has 4 million times the mass of the Sun, but is not currently feeding, so it's not active).
Also, we're not completely sure what the structure of the accretion disk is like near a hole. The current theory is that near the black hole it's very flat and thin, but farther out it puffs up into a torus or doughnut (or bagel if you're from New York City). But think about this: imagine putting a pea in the center of a donut hole. From most viewing angles, the pea is hidden. If you view it face-on you can see the pea, but at an angle the donut blocks your view. From edge-on you're looking through a lot of doughnut and can't see the pea at all.
This model explains a lot about what we see with these active galaxies, but is it right?
Maybe. Maybe not.
The new observations from Chandra are very interesting. If we see a black hole torus face-on, we expect to see X-rays of all energies, since they are free to get to us. But if we see one edge-on, only the highest-energy X-rays can penetrate the obscuring torus, so we'd expect to see only those high-energy X-rays and no low energy ones. So, looking at 1300 black holes as Chandra did, you'd expect to see a few that are face-on, a few edge-on, and most ranging in between. In other words, the observations should show most black holes sending out a mix of high and low energy X-rays.
Oops. They didn't. They reveal lots of high-energy-X-ray-emitting-galaxies, and lots of low-energy-X-ray-emitting-galaxies, but very few in between, the opposite of what the model predicted.
Does this mean the model is completely wrong? No, because in fact the model does very well predicting what we see from black holes in a whole bunch of other obsrevations, hundreds and even thousands of them. So what these new data really mean is that the details of the model need to be worked on more. Maybe in active galaxies the torus doesn't puff up as much. Maybe the disks are bigger than we thought, or there isn't as much dust in the torus, or a hundred other reasons.
The devil, that rascal, is always in the details. And if there is any actual place in the Universe that could be described as Hell, it's the gaping maw of a black hole and the swirling maelstrom surrounding it. So there will always be devilish details to hammer out.
One final note: these active galaxies can pour out gamma rays as well-- gamma rays have even higher energy than X-rays. NASA and the Department of Energy are building GLAST, an observatory whose main mission is to investigate these supermassive black holes (I've written about GLAST several times). It's due for launch in November, so by this time next year we'll have a lot more data, and a lot more answers. But we'll have a lot more questions, too! This is truly the game that never ends, which is one reason it's so much fun.