Previously, it was believed that a galaxy's central bulge was a definite indicator of a central black hole. A recent discovery made by the Spitzer Space Telescope may change this notion, however. While galaxies come in many shapes and sizes, the central bulges of some spiral galaxies were attributed to black holes. The Spitzer Space Telescope, in its recent survey of 32 flat, bulgeless spiral galaxies, found 7 galaxies harboring massive black holes. This, of course, defies the notion that a spiral galaxy's central bulge is related to its central black hole.
In the past, it was thought that the size of a galaxy's central bulge was an indicator of the size of its central black hole. At the time, observations had approximated the mass of a central black hole to be about 20% of the mass of the galaxy's bulge. It would then follow that a galaxy with a huge central bulge would have a huge central black hole. Spitzer has now shown this hypothesis to be not entirely true, however.
So if the central bulge is not an indicator of black holes, then what is? It turns out that black holes aren't truely and entirely "black". Matter emits high-energy photons as it crosses the event horizon. These high-energy photons energize the surrounding gas much more strongly than any other source could, which is detectable by the Spitzer Space Telescope. In the seven flat galaxies with black holes in them, Spitzer found highly energized neon gas, a sure indicator of a nearby feeding black hole.
Recent speculation of the existence of dark matter may be of importance here. If the bulges aren't a necessary ingredient of the black hole recipe, then something must be forming the black holes, right? Dark matter, which can't be seen but whose gravitational effect can be felt, is said to be responsible. Could this be? Or is it physicists grabbing at something to explain the unexplained, the next luminiferous ether? It is current belief that dark matter could account for up to 90% of a galaxy's mass, accounting for seemingly "missing" mass, and therefore, account for the formation of such black holes.