But this May, NASA revealed data from its Hubble Space telescope providing the best proof yet that black holes are indeed real.
The telescope measured the velocity of gases swirling closely around the center of M87, an elliptical galaxy in the Virgo cluster, about 50 million light-years away from Earth.
The gases were found to be spinning so quickly that only an unusually massive object at the galaxy's center could exert enough gravity to keep them in orbit.
According to the current theories of physics, only a black hole is sufficiently massive to exert such an attractive force.
"[The telescope's data] is very significant because it nails down in a very definite way something that had been widely believed," says professor of Astronomy and Physics William H. Press' 69 "I think now it's almost impossible to doubt the conclusion that it's a black hole."
But not all scientists agree about the finality of the Hubble finding, which reveals only the existence of a large mass in a small space.
"It could be a big bowling ball in there for all we know but we don't know of any bowling balls of a billion solar masses," says Professor of Astronomy Robert P. Kirshner '70.
Very Dense Objects
Black holes are really not holes at all, but rather dense objects with incredibly strong gravitational fields from which even light cannot escape.
While most dying stars evolve into planetsized white dwarfs stars with masses greater than three times that of the sun explode as supernovae and then crush themselves under their own gravity to become black holes.
No one supernova, however could have formed the black hole found by the Hubble telescope.
Because it has the mass of about 2 billion suns and spans the size of the solar system, Kirshner says the black hole is termed "supermassive."
Kirshner, who chairs the astronomy department, himself studies supernovae using the Hubble telescope most recently one in the Whirlpool galaxy.
Supermassive black holes may arise from a whole cluster of stars collapsing at the center, he say.
Professor of Astronomy Margaret J. Geller, who teaches Astronomy 14, "The Universe and Everything," says the black hole could have accumulated such a great mass by "gobling" up nearby objects for a few billion years.
But Kirshner says scientists don't have a conclusive explanation for how black holes of this magnitude first form.
"I would say that this is something that people really don't know," says Kirshner. "If [supermassive black holes] turn out to be common, then we better come up with a good story."
Since black holes do not emit or reflect light by definition, and therefore cannot be seen, their existence is based on indirect evidence, say Kirshner.
"You argue that there's a black hole there because of the motions that it causes," he says.
swirling around the center of the M87 galaxy are various fast-moving gases whose speeds can be measured by comparing the shifts in the wavelength of light emitted from the approaching and receding sides of the rotating disk of gas.
Knowing the velocity of the gas, which was calculated to be about 1.2 million miles per hour, Kirshner says simple Newtonian physics was used to compute the mass necessary to provide sufficient gravity to keep the disk of gases from flying apart.
And such data showed that there was an unusual amount of mass in a relatively small volume-the telltale sign of a black hole.
Similar measurements had been conducted by ground-based telescopes, but the Earth's atmosphere blurred the image considerably.
The advantage of the Hubble telescope, says Press is that it orbits above the Earth's atmosphere and can therefore measure the speeds of gases much closer to the galaxy's nycleus. "You get much better resolution," he says.
In essence the Hubble telescope made an existing argument stronger, since scientists using similar techniques had already suspected the presence of a black hole in the M87 galaxy.
The telescope data "showed that the mass must be larger...and contained in a smaller region" than previously calculated using ground based telescopes, Press says.
Even simple pictures taken by the Hubble suggest the existence of a black hole.
"There's a very sharp concentration of light in a small place in the center of the galaxy as if there was some object there keeping stars in a close orbit," Kirshner says.
The black hole observed by the Hubble telescope also emits narrow jets of electrons traveling at nearly the speed of light and extending light years into space.
Geller says such "radiation being shot out of a firehose" is consistent with a black hole absorbing neighboring celestial objects and squeezing out their energy.
But not everyone is as convinced as Press that the Hubble finding is actually a black hole.
Professor of Astronomy Ramesh Narayan says that different astronomers might interpret the results differently.
"It's an interesting observation," says Narayan. "I think you'll hear different reactions on whether it's the greatest breakthrough or just another piece in the puzzle.'
Like Narayan, Kirshner has doubts about the finality of the Hubble findings.
"Here's the tricky part. There's not anything they've measured that tells you that there's a black hole, Kirshner says. "What the measurement tells you is that it's a large mass in a small space".
Kirshner says the phenomenon is only explained as a black hole by default because astronomers don't know of any other theory that fits the data.
And since a black hole is a theoretical construct that cannot be seen, an absolute proof is going to be difficult to find.
Geller is equally cautious. The black hole conclusion is "a reasonable interpretation of the data," but a confirmation is an overstatement, she says.
"It's not an acceptable proof. Just because people can't think of [something other than a black hole] doesn't mean nature can't Geller says. "There are a lot of things that people don't understand that nature has been doing for some time."
The next step toward understanding the nature of black holes-and perhaps providing stronger proof of their existence to skeptics-is to look for phenomena similar to the M87 gas mass in other galaxies, scientists say.
"Some people think that every galaxy at its nucleus harbors a black hole," Geller says.