Pirius knew, in theory, about dark matter. It was an invisible shadow of normal matter, the “light” matter made of protons and neutrons. The dark stuff interacted with normal matter only through gravity. You couldn’t burn it, push it away, or harvest it, save with a gravity well. And it was harmless, passing through light matter as if it weren’t there. Pilots and navigators were taught to recognize its presence; sometimes great reefs of the stuff could cause gravitational anomalies that might affect your course.

Aside from that, dark matter was of no consequence. Pirius couldn’t see why anybody would study it.

But Faya showed them Virtuals. Sol system had coalesced out of a disc of material that had once stretched much farther than the orbit of the farthest planets. Most of the mass of the disc was now locked up in the bodies of the planets, but if you smeared out the planets’ masses, you got a fairly smooth curve, showing how the mass in the disc had dropped off evenly as distance from the sun increased, just as you’d expect.

“Until you get to Neptune,” Faya said. At the rim of the Kuiper Belt the actual mass distribution plummeted sharply. “There are many bodies out here, some massive. Pluto is one, Port Sol another . . . But they add up to only about a fifth of Earth’s mass. There should have been thousands of worldlets the size of Pluto or larger. Something removed all those planetesimals—and long ago, when Sol system was very young.”

She summarized theories. Perhaps the missing worldlets had been thrown out of their orbits by the migration of a young Neptune through Sol system, as it headed for its final orbit. Perhaps there was another large planet, out there in the dark, disturbing the objects’ orbits—but no such planet had been found. Or maybe a passing star had stripped the Kuiper cloud of much of its richness. And so on.

Pirius said, “None of that sounds too convincing.”

Faya Parz said, “If mankind has learned one thing in the course of its expansion to the stars, it is that the first explanation for any unlikely phenomenon is life.”

Luru Parz had come to this place to study the traces of that ancient plunder. Her first theory was that it could have something to do with dark matter. Dark matter was relatively rare in the plane of the Galaxy, and indeed in the heart of Sol system. “But it is to be found out here,” Faya said, “where the sun is remote, and baryonic matter is scarce.”

Pirius tried to put this together. “And you think there is life in the dark matter. Intelligence.”

“Oh, yes.” Faya’s eyes were hooded. “There is six times as much dark matter as baryonic in the universe. Everywhere we look, baryonic matter is infested with life. Why not dark matter? In the past, humans have studied it. We have some of the records. Luru even believes that a conflict between intelligences of dark and light matter is underway—an invisible conflict more fundamental even than our war with the Xeelee. The Qax destroyed much of our heritage, but there are hints in the surviving pre-Occupation records—”