led to unprecedented precision and deeper-than-ever sky surveys. With these electronic spectacles, once-faint blurs have revealed themselves as extremely distant galaxies that can be analyzed and cataloged. Masterful computer algorithms piece together terabytes of photonic information into detailed three-dimensional images of space. Consequently, for the very first time, astronomy has added realistic depth to its spatial maps.

A leading ground-based project, called the Sloan Digital Sky Survey, has employed these state-of-the-art techniques in a comprehensive three-dimensional scan of a large portion of the northern sky. Mapping more than 200,000 galaxies, the survey has dramatically increased our knowledge of vast segments of space.

Paradoxically, though these instruments and programs have provided more information about the universe than ever before in scientific history, they have revealed how much we really do not know. In particular, they have confirmed a gnawing suspicion among cosmologists that the vast majority of the universe is composed of invisible materials and unidentified energies. As the telescopic results have indicated, only a small fraction of the mass of the cosmos constitutes ordinary matter. The rest is terra incognita! Not only do unseen powers appear to dominate space, they seem to govern its overall dynamics—causing the universe to expand at an ever-increasing rate. In short, we appear to live in an accelerating universe fueled by a hidden dynamo of mysterious origin.

This extraordinary discovery sent shock waves through the world of cosmology, displacing a number of long-held conceptions. No longer can cosmologists focus on the simplest models with the most basic kinds of matter—the textbook examples of expanding universes. Rather, the new findings have revealed more unusual possibilities and solutions.

Some of these novel proposals hypothesize strange new substances with properties unlike anything ever seen. Could, for instance, objects exist with negative mass? Could there be shadow worlds able to communicate with us only through the pull of gravity?

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