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Ordinary matter: Lost and found

Never mind about dark matter. Forget dark energy. Astronomers aren't even sure of the whereabouts of most of the cosmos' ordinary material: protons, neutrons, and electrons. New findings add to the evidence that two-thirds of this matter resides not in galaxies but in warm gas clouds that surround them.

Ordinary matter has posed a puzzle for many years. Observations of the early universe indicate that ordinary matter should account for 4 percent of all mass and energy, with dark matter and dark energy making up the rest. Yet astronomers looking for normal matter in galaxies have found only about one-third of this amount.

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