Introduction:What the Eye Can't See
One of the most remarkable astronomical revelations of our time is the discovery that the universe is expanding. But that revelation has left us with equally great questions: Will the outward rush continue forever, even to the point where the stretching of space rips galaxies apart? Or will the motions reverse at some point so that the firmament collapses back into a big crunch?
In principle, determining our fate should be a simple matter of cosmic bookkeeping. The answer depends on how much mass is out there: Gravity, which attracts objects toward each other, fights against expansion. By tallying up everything we can see, astronomers should be able to predict if there’s enough stuff out there to pull the cosmos back together. Yet every time they attempted a cosmic census over the past 70 years, the results came out wildly inconsistent. Eventually, the failures led to a profound realization: The numbers never seem to add up because the vast majority of the universe cannot be seen. As the fox says in Antoine de Saint-Exupéry’s beloved book LePetit Prince, “L’essentiel est invisible pour les yeux”—the essential is invisible to the eyes.
The standard tools of astronomy cannot probe this dark portion of the universe. Optical telescopes are wonderful at spotting stars, which radiate most of their energy as visible light, but these telescopes are blind to anything that does not shine. Instruments that pick up other frequencies of the electromagnetic spectrum—radio waves or X rays, for instance—also cannot detect the shadow universe. So in recent years researchers have learned to use indirect observations. Measurements of the bending of light, the motions of galaxies, and the brightness of distant exploding stars have revealed a new truth: Unseen elements, collectively called dark matter and dark energy, account for roughly 96 percent of the mass of the universe. Stars are the extreme minority, making up just 0.4 percent of the total.
All the smudges and stipples of light in the night sky, the bits of the universe we have studied since antiquity, are just tiny flecks of foam on a huge, dark cosmic sea. Until we solve this mystery, we cannot truly understand where we came from or learn what the future has in store for us.
