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Brown dwarfs—if they indeed exist—are celestial objects composed of dust and gas that failed to evolve into stars. To be a star, a ball of hydrogen must be large enough so that the pressure and heat at its core produce nuclear fusion, the process that makes stars bright and hot. Brown dwarfs, so named by American astronomer Jill Tarter in 1975, range in mass between the most massive planets and the least massive stars, about 0.002 to 0.08 times the mass of the Sun.

Roughly 90 percent of the material in the universe is unaccounted for. Since it cannot be seen, this substance is called dark matter. The existence of dark matter is confirmed by the fact that its mass affects the orbits of objects near the visible edge of galaxies and of galaxies within clusters of galaxies. If brown dwarfs really are as common as astronomers think, their total mass could account for the mass...