Keith lowered himself to the ground again, which, at this spot, was covered with clover. Keith found it quite comfortable to sit on. He ran his hand through the clover, enjoying the feel of it against his skin, and looked around. It was a remarkable Simulation, he thought.

So relaxing, so beautiful.

He watched some birds moving high overhead, but they were too far away for him to identify the species.

Keith plucked a piece of clover and brought it up to look at. Maybe this was his lucky day; maybe he'd find a four-leaf clover…

What luck. He did.

He plucked a few more pieces, and his jaw dropped.

He pressed his face to the ground, and examined plant after plant.

They were all four-leaf clovers.

He brought one up to his face, held between thumb and index finger, and scrutinized it. It seemed like normal clover in almost every way. It even bled a little green plant juice from its severed stem. But each of these clovers had four leaves. Keith remembered from undergraduate botany that the genus name for clover was Trifolium — three leaves. By definition, clover had three leaves, except in the odd mutant individual. But these plants all had four distinct oval leaves.

Keith looked at the white and pink flowers growing from some of the plants. Definitely clover — but four-leaf clover.

He shook his head. How could Glass have gotten all the 'other details right, but have made a mistake such as this? It didn't make any sense.

He looked around again, searching for any other discrepancies.

Most of the deciduous trees did indeed seem to be maple — sugar maple, in fact, if he wasn't mistaken. And those conifers were jack pine, and the big one a little farther along was a blue spruce. And — And what kind of bird was that? Sitting in that blue spruce? Surely not a cardinal or ajay. Oh, it had the tufted head crest, but it was emerald green, and its bill was flat and spatulate, unlike that of most songbirds.

It was Earth; no doubt about it. That was Earth's moon, still sitting high in the daytime sky. And yet, it wasn't quite Earth — some of the details weren't right.

Keith chewed at his lower lip, puzzled…

Jag and Rissa took an elevator up to the bridge, and soon the Waldahud was standing in front of the two rows of workstations, telling his colleagues of the fantastic discovery.

"There's a metaphor that's been carried by the current for years," he barked, "that visible matter is just froth on an inky ocean of dark matter. We knew the dark matter was there because of its gravitational effects, but we've never seen it until now. Those spheres out there, and the gravel fog between them, are made out of dark matter."

Lianne let out a low whistle. Keith raised an eyebrow. He knew a bit about dark matter, of course. CalTech astronomer Fritz Zwicky had deduced its existence back in 1933, through observations of the galaxies in the Virgo Cluster.

Those galaxies were rotating around each other so quickly that if the visible stars were the only major source of mass present, the whole thing should have flung apart long ago. Subsequent studies showed that almost every large structure in the universe — including our Milky Way galaxy — behave as if there were far more mass present than could be accounted for by the suns and any reasonable number of attendant planets. Some previously undetected matter, dubbed "dark matter" because it was apparently neither luminous nor highly reflective, accounted for over 90 percent of the gravity in the universe.

As usual, Thorald Magnor had his large feet up on his console, and his thick fingers interlaced behind his head, buried in his red hair. "I thought we'd already discovered what dark matter was," he said.

"Only part of it," said Jag, lifting two of his four hands.

"We've long known that baryonic matter — matter made up of protons and neutrons — accounts for less than ten percent of the mass of the universe. In 2037, we discovered that the ubiquitous tau neutrino has a very slight mass — about seven electron volts' worth. And we found that the muon neutrino also has a trifling mass, about three one-thousandths of an electron volt. Since these two types of neutrinos are so abundant, in total they account for about three or four times more mass than all the baryons do. But that still left us with as much as two thirds of the universe's mass unaccounted for — until now."

"What makes you think the stuff out there is dark ' matter?" Keith asked.

"Well," said Jag, "it isn't normal matter; that much is certain."

Although he was trying to hide it, Jag was holding on to the beveled edge of Thor's console with one hand so that he wouldn't drop down onto four legs. Starplex operated on a four-shift cycle as a concession to the Waldahudin, who came from a world with a short day, but Jag had been working overtime. "In early dark-matter studies, there were two candidates for the material composing it, named WIMPs and MACHOs by human astronomers — all of whom should have to swim in a river of urine, by the way. WIMPs are 'weakly interacting massive particles' — you see the gibberish foisted upon us in search of these silly acronyms? Anyway, the tau and muon neutrinos turned out to be WIMPs."

"And MACHOs?" asked Keith.

"'Massive compact halo objects,'" said Jag. "The 'halo' is the sphere of dark matter that has a galaxy at its center.

The 'massive compact objects' were thought to be billions of Jupiter-sized bodies not associated with any particular star — a fog of gaseous worlds through which the luminous material of the galaxy moves."

Lianne was leaning forward, chin resting on her hand. "But if the universe really were permeated with — with MACHOs," she asked, "wouldn't we have detected them by now?"

Jag turned to her. "Even Jupiter-sized objects are puny on the cosmic scale. And since they're nonluminous, the only way we would see them is if one wandered in front of a star we happened to be observing.

Still, the effect would be minor: just a slight gravitational lensing of the star's light, causing a temporary brightening. Such events have occasionally been seen; the oldest recorded observation of one was made by human astronomers in 1993. But even if space were lousy with MACHOs — enough so that they made up two thirds of all the mass in the universe — only one out of every five million stars you could observe at any given moment would likely be undergoing gravitational lensing due to one passing by." He gestured toward the twinkling part of the starfield. "We only see gross effects here because we're so close to the field of dark matter, and because the dark matter itself is transparent.

We're actually just seeing regular space dust, sprinkled throughout the dark-matter objects."

Keith looked at Rissa, his eyebrows raised. She made no objection.

"Well," said the director, "this certainly seems to be a major discovery, worthy of further-"

"Forgive the interruption," said Rhombus, "but I'm detecting a tachyon pulse." Rhombus rotated the starfield hologram surrounding the bridge to bring the shortcut front and center; the effect on Keith's stomach was similar to what he experienced in a planetarium when the operator was trying to demonstrate that learning could be fun. Jag quickly took his seat on Keith's left. The shortcut was a pinprick of green — the color of whatever was coming through it — surrounded by the usual ring of violet Soderstrom radiation.

"Is it a Commonwealth ship?" Keith asked.

"No," said Rhombus. "There's no transponder signal of any kind." The green spot continued to grow. "Incredulous: that is bright" — PHANTOM's stilted translation of the words that were flashing over Rhombus's mantle. But the Ib was right. The shortcut was the brightest object in the sky? exceeding even the A-class star Jag had spotted earlier.