One of the most common questions that I get from my astronomy students is "how do we know black holes exist if we can't see them?" It's a great question, and the short answer is that we have to be clever by looking at how the black holes effect the space around it.
Usually, scientists look for the gravitational effects of black holes on companion stars, or bursts of energy coming from superheated mass as it falls into the gravitational well. However, this concept is sometimes difficult for students to get their heads around, so I went looking for visual evidence and stumbled upon the above image on NASA's site.
What, your not impressed? Alright, let me walk you through it. Back in 1997 the Hubble Space Telescope turned its eye to the galaxy M84, seeking evidence of a supermassive black hole at its core.
Using its onboard spectrograph, Hubble was able to image the transverse velocity of the gas around the galaxy's core. In the absence of a strong gravitational field the colored line down the center would be straight. However, it is clearly warped in the center of the image.
Expressed numerically, the gas within a 26 light-year radius around the center of the galaxy was traveling at speeds in excess of 880,000 miles per hour. To induce such speeds a massive body at least 300 million times the mass of our Sun would have to be present.
While such an image is not as "impressive" as the mental images we often create when pondering exotic objects like black holes, it does provide empirical evidence of their existence.
Interested in more black hole pictures? Check out our black hole image gallery.
Image Credit: NASA, Gary Bower, Richard Green (NOAO), the STIS Instrument Definition Team