A smattering of small galaxies appear to be missing a whole lot of dark matter.
Most of a typical galaxy is invisible. This elusive mass, known as dark matter, seems to be an indispensable ingredient for creating a galaxy — it’s the scaffolding that attracts normal matter — yet reveals itself only as an extra gravitational tug on gas and stars.
But now, researchers have found 19 dwarf galaxies — all much smaller than the Milky Way — that defy this common wisdom. These newly identified outliers have much less dark matter than expected. The finding, published November 25 in Nature Astronomy, more than quintuples the known population of dark-matter renegades, adding fuel to an already simmering mystery.
“We are not sure why and how these galaxies form,” says Qi Guo, an astrophysicist at the Chinese Academy of Sciences in Beijing. Typical dwarf galaxies concentrate dark matter far more than their larger cousins, she notes. Their smaller size leads to weaker gravity, which has trouble holding on to tenuous clouds of gas. That usually shifts the balance of mass in dwarf galaxies away from normal matter and toward dark matter.