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.

Dispute

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.