Albert Einstein hinted at the existence of a mysterious force like this when he proposed the concept of a cosmological constant. After Hubble made his observations, Einstein repudiated the constant as the greatest blunder of his career. Now it looks as if he spoke too soon.

But cosmologists admit they have no clue what dark energy really is. It could be Einstein’s constant, or a form of anti-gravity that changes over time, or evidence for string theory, the idea that the universe is composed of tiny strings and dimensions we can’t see.

Finding the answer has become one of NASA’s top priorities. The Joint Dark Energy Mission, an effort involving NASA and the Energy Department, seeks to send a new telescope into space tailored to find out more about dark energy. So far, three candidates have emerged: the Dark Energy Space Telescope, or Destiny; the Advanced Dark Energy Physics Telescope, known as ADEPT; and the SuperNova Acceleration Probe, or SNAP.

All would detect thousands of Type Ia supernovae. ADEPT would measure ancient sound waves that rumbled through the early universe, leaving tiny variations in how galaxies are distributed. SNAP and Destiny would use gravitational lensing to measure how the distribution of matter across the universe could be affected by dark energy.

Charles Bennett, a professor of physics and astronomy at Johns Hopkins University who is leading the ADEPT team, said the mission would probably be selected by the middle of next year, and the telescope would be in space a decade from now.

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