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."

Indirect Evidence

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.

More Evidence

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.