for National Geographic News
The nearly two dozen dwarf galaxies known to orbit the Milky Way vary greatly in brightness, from a thousand times to ten million times the luminosity of the sun.
But at their cores, all of them have about the same mass, according to new work that sheds light on how the enigmatic substance known as dark matter helps galaxies form.
This mystery matter is thought to make up the majority of mass in the universe but is invisible except for its gravitational influence on ordinary matter.
Using the relative speeds of stars in the Milky Way's satellite galaxies, a team led by Louis Strigari of the University of California, Irvine (UCI) found that 18 of the 23 known satellite galaxies have a common central mass of about ten million times that of the sun.
That means dark matter is the most likely source of the unexpected mass in the faint galaxies. It also suggests that dark matter can provide the minimum mass required for galaxies to form.
"We've gone down to the smallest galaxies we can see," said Manoj Kaplinghat, a UCI astrophysicist and study co-author.
"What's surprising is there's so much dark matter, even though these guys are little. They barely have a few thousand stars."
Dark Milky Way
Research over the past two decades has revealed that dark matter provides structure throughout the universe.
Scientists believe that galaxies form as dark matter's gravity attracts normal matter, creating the well-known groupings of gases, stars, and other objects. They also suspect that small galaxies merge over time to create larger galaxies such as our Milky Way.
"Basically galaxies like our own wouldn't have formed if we didn't have dark matter," Kaplinghat said.
Currently the ratio of dark matter to visible material in the Milky Way is about ten to one, he added.
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