Stars in the Milky Way account for only a few percent of our galaxy’s mass. According to the consensus picture of galaxy formation, most of the mass in the Milky Way—and every other galaxy—is housed in a sphere of dark matter, the nonluminous, feebly interacting substance whose identity is still a mystery. Now Pieter van Dokkum of Yale University and his international team have identified a galaxy, NGC 1052-DF2, that challenges the standard formation picture: Its mass in dark matter is at most about the same as its mass in stars, and possibly it is much less.

The figure shows a Hubble Space Telescope image of the faint galaxy; the 10 labels indicate star clusters on the outskirts. From Doppler-shift measurements of each cluster’s absorption spectrum, the researchers could obtain the speed at which the cluster orbits the galaxy.

Just as, in our solar system, the mass of the Sun...

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