To quote Holly from Red Dwarf: "The thing about a black hole - its main distinguishing feature - is it's black. And the thing about space, the colour of space, your basic space colour, is black. So how are you supposed to see them?"
It turns out it's rather easy, if you can see in x-rays: matter is heated as it falls onto a black hole, which causes x-ray radiation that we can detect. We also see "jets" of radiation coming out like beams from a lighthouse, which it has long been assumed is due to charged particles like electrons being forced out at close to the speed of light.
Now, we've got some good evidence that the jet model is indeed plausible. In work recently published in Nature, a team of researchers has shown that hydrogen gas in the radio jets of a galaxy called IC 5063 is moving at high velocities (up to about 600 kilometres per second). This suggests that the molecules have been accelerated by charged particles, as theorized.
But how do black holes accelerate this cold hydrogen gas to hundreds of thousands of miles per hour? Until now, that has been hard to fathom.