Tiny Satellite Galaxies May Help Solve Dark Matter Mystery


Hubble observations of the smallest galaxies known help explain why so few of them exist

As galaxies go, these are a bunch of wimps. The so-called ultrafaint dwarfs are like ghost galaxies—tiny wisps against the sky that were first discovered less than a decade ago. Now about a dozen have been found that orbit our Milky Way galaxy, and they’re helping to solve an astronomical riddle called the missing satellites problem.

Dark matter is an intangible, invisible substance thought to make up 23 percent of the universe, and models suggest that it should be at the heart of thousands of mini galaxies orbiting the Milky Way and other large spirals. Yet only a few dozen have ever been observed, counting both the ultrafaint dwarfs and the not-quite-as-small dwarf galaxies. The mismatch between hypothesis and observation has caused some physicists to question dark matter models’ prediction of the existence of so many small galaxies. Now, new Hubble Space Telescope observations of some of the tiniest Milky Way satellite galaxies are offering clues about where the rest of their siblings may be hiding.

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