One of the most pressing questions: How did this teeming array of galaxies get winnowed down to the population astronomers see now in the nearby universe? Did they collide to form larger galaxies, or did many simply fade away?

"The problem is we can't observe the evolution of an individual object," Ellis explains. "All we can do is piece together galaxies seen at different times." It’s a bit like seeing a tank full of tadpoles and frogs, without being able to watch one transform into the other.

For example, astronomers have long debated the origins of elliptical galaxies, and whether they might somehow be related to spiral galaxies. At one time, Ellis says, astronomers thought spirals and ellipticals were two entirely different species that evolved independently.

"It seemed inconceivable that one galaxy like a spiral could morph itself into an elliptical," Ellis says. "But we now think there are occasions when this actually happens." Some ellipticals may form after two spirals collide. And some, says Ellis, may have started out as ellipticals and stayed that way.

That certainly seems to be true of quasars, brilliant beacons of the early universe thought to be young galaxies. The intense brightness of quasars is thought to result from matter falling into a supermassive black hole at the quasar's center.

But quasars have turned out to be quite rare in comparison to other protogalaxies seen during the same epoch of cosmic history, and it's unclear how quasars fit into the scheme of galactic evolution. "The question," says Ellis, "is are quasars some freak phenomenon, and should we therefore put them aside? Or should we be joining them up [with other protogalaxies] in some kind of cosmic evolutionary cycle?"

As for the question of black holes contributing to galaxy formation, Sandy Faber points out that there are some sticky questions yet to answer concerning the physics of the black holes themselves. "Before I would worry about black holes and their relationship to galaxies," Faber says, "a prior question, and I think most people would agree, is how you get the matter down into the hole."

Ellis adds, "No question that it happens. The question is how frequently: Is it a fundamental part of the jigsaw puzzle of galaxy formation, or is it a sideline?"

With a new generation of giant telescopes planned for use in space and on Earth, astronomers may be on the verge of answering some of these questions. "It's a very data-rich subject at the moment," Ellis says, "and it's going to get more exciting."