Now astronomers have discovered that small, nearly invisible galaxies may, in fact, be heavyweights, with far more than their expected share of dark matter. And, since these dwarf galaxies are so common, they could make up a substantial fraction of the total mass of the universe.
"Although these [dwarf] galaxies contain almost no stars, their density of dark matter is very high," said John Kormendy of the University of Hawaii. Kormendy, who worked with Kenneth Freeman of Australia's Mount Stromlo Observatory, gave a presentation this week at the winter meeting of the American Astronomical Society in Austin, Texas.
Kormendy and Freeman studied 43 galaxies, ranging from the brightest to the just barely visible. They described the faintest galaxies as "gossamer" in appearance - because they produce so little light, they hardly show up even in time-exposure photographs of deep space, taken with the largest telescopes. But their high content of dark matter means these tiny galaxies could actually outweigh their brighter siblings.
"It wouldn't take many undiscovered galaxies to make them add up to an important amount of mass," Kormendy said. He pointed out that our own local cluster of galaxies contains just two big, bright galaxies - our Milky Way, and the Andromeda Galaxy. But it contains several medium-sized galaxies, and dozens of extremely faint ones.
Astronomers have long puzzled over the problem of dark matter. They've suggested it could consist of massive particles left over from the big bang - or perhaps "brown dwarf" stars that are bigger than the planet Jupiter but too small to ignite a nuclear reaction in their cores. Various other candidates have also been put forward, including neutrinos and black holes.
These dwarf galaxies are also believed to be extremely old. As relics of the early universe, they may help astronomers to shed light on the process of galaxy formation.
Vera Rubin, a pioneer in the study of dark matter at the Carnegie Institute in Washington, says that these small galaxies - rich in dark matter, but giving off almost no light - may be extremely common.
"What we've learned in the last half-century is that nature tends to do anything that is not forbidden," she remarked. But it still may be decades before the problem of dark matter is unravelled, she said.
Rubin compared the astronomers' challenge to that of putting together a completely black jigsaw puzzle in a completely dark room. "We don't know where all the pieces are, and when we do find a piece, we're not absolutely sure where it fits."