What's in the dark? Countless generations have gazed up at the night sky and asked this question--the same question that cosmologists ask themselves as they study the universe.

The answer turns out to be surprising and rich. The space between stars is filled with an exotic substance called "dark matter" that exerts gravity but does not emit, absorb, or reflect light. The space between galaxies is rife with "dark energy" that creates a sort of cosmic antigravity causing the expansion of the universe to accelerate. Together, dark matter and dark energy account for 95 percent of the...