Black holes, for example, once fodder for science fiction, now are of high interest in observational astronomy. But how can scientists observe something that by its very nature cannot be seen?

Hubble has provided the first direct evidence that our universe includes objects so dense with matter that not even light can escape their gravitational pull. It has imaged discs of matter warped by invisible forces, opening new inquiries about how black holes are formed and what happens to them.

The telescope has traced gamma-ray bursts to distant galaxies, but it sheds no light on what is causing these strange explosions that for a few seconds release extraordinary amounts of energy. Could it be the collision of galaxies? The death of a black hole?

"We are in a renaissance for astronomy," says NASA's associate administrator for space science, Ed Weiler. "Sometimes it is more important to know the things we don't know."