Most of this made no difference to the dark matter sea... but, here and there, the material of the shining stars began to exert an influence on its dark counterpart. Just as baryons had slithered into dark matter potholes, so -- on a much smaller scale -- photinos collected in the pinpoint gravity wells of the new stars.

Even the human star, Sol, had contained a dark core the size of a moon. Human scientists had observed this dark parasite indirectly by its effect on the neutrino output of the Sun...

And, in a slow explosion of insight, Paul began to see a connection between the dark matter canker at the hearts of the stars, and the aging of the baryonic Universe.

Excited, he skimmed through the Universe, studying the cooling corpses of extinguished stars.

And at last, datum by datum, he came to understand the secret history of the Universe.

Thanks to the baryonic stars small-scale structure entered the dark matter Universe. Paul speculated that a chemistry must have begun, with varieties of the photinos combining to form some counterpart of molecules; strange rains had sleeted over the surfaces of the shadow worlds, still buried in the blazing cores of baryon stars.

At last life had arisen.

Paul had no way of knowing if the transition to life had occurred on one of the shadow planets or on several, perhaps in a variety of forms. Nor could he guess what form that life had taken, what technologies and philosophies it had evolved.

But he could speculate how it had spread. Photino creatures like birds -- photino birds -- had fluttered out through the baryon stars as if they did not exist, colonizing shadow world after shadow world. Perhaps, Paul supposed, vast flocks had plied between the hearts of stars, with the humans and other baryonic races all unaware.

Aeons had passed with the two grand families of life, dark and light, oblivious of each other...

Then something had happened.

Again Paul could only guess. Probably a supernova had ripped apart a baryon star, laying waste to its host shadow world in the process. Paul imagined the horror of the photino civilization as the irrelevant froth of baryons through which they moved turned into a source of deadly danger, perhaps threatening the ultimate survival of their civilization.

Many courses of action must have been considered, including -- Paul speculated with a kind of shudder -- the total annihilation of the baryonic content of the Universe. But without baryon stars and their tiny gravity wells new shadow worlds could not form; therefore without the baryons there could be no replacement for the photino worlds as they grew stale and died: and so, in the end, the dark civilization itself would falter and fail.

So the baryons had to stay. The photino birds needed the stars.