Meanwhile, astronomers are confronting another vexing challenge, and it's a whopper. Put simply, we don't know what 96 percent of the universe is made of. All of the galaxies, stars, gas, planets, and people combined make up just 4 percent of the contents of the cosmos. We've given the rest of the recipe the simple but sinister names of dark matter (25 percent) and dark energy (71 percent). Together they present the biggest mysteries in astronomy and physics, perhaps in all of science.

Dark matter is somewhat easier to picture. The motions of galaxies, both the way they revolve and the way they cluster in groups, imply that a major hidden source of gravity binds everything together. Astronomers believe every galaxy sits within a cocoon of dark matter that is far larger and heavier than the galaxy itself. Computer models of the universe predict a vast cosmic web of dark matter, a network of filaments within which galaxies are embedded like fireflies in a spider's lair.

This web does some odd things. For instance, a ray of light traveling through pockets of dark matter gets bent to and fro along the way. When a telescope takes images of remote galaxies, their shapes and positions are distorted— usually just slightly, but sometimes severely—by this effect. It's like looking through the wavy glass of the door on a shower stall. Nothing in the distant universe is quite as it appears.

Scientists think dark matter is made of unidentified particles that rarely interact with ordinary matter. They've built experiments to try to catch such particles, but so far dark matter has defied detection.

When it comes to dark energy, scientists don't have a clue. It's some substance or property of space itself that endows space with a negative pressure. It acts in a sense like antigravity, accelerating the growth of the cosmos with increasing urgency. As the entire universe expands—a process set in motion by the big bang 13.7 billion years ago—the influence of dark energy grows ever larger. It's disquieting, for scientists and astronomy fans alike.