Dark-Matter Galaxy Detected: Hidden Dwarf Lurks Nearby?

An entire galaxy may be lurking, unseen, just outside our own, scientists announced Thursday.

The invisibility of "Galaxy X"—as the purported body has been dubbed—may be due less to its apparent status as a dwarf galaxy than to its murky location and its overwhelming amount of dark matter, astronomer Sukanya Chakrabarti speculates.

Detectable only by the effects of its gravitational pull, dark matter is an invisible material that scientists think makes up more than 80 percent of the mass in the universe. (See "Dark Matter Detected for First Time.")

Chakrabarti, of the University of California, Berkeley, devised a technique similar to that used 160 years ago to predict the existence of Neptune, which was given away by the wobbles its gravity induced in Uranus's orbit.

Based on gravitational perturbations of gases on the fringes of our Milky Way galaxy, Chakrabarti came to her conclusion that there's a heretofore unknown dwarf galaxy about 260,000 light-years away. (Related: "Huge Black Hole Found in Dwarf Galaxy.")

With an estimated mass equal to only one percent the mass of the Milky Way, Galaxy X is still the third largest of the Milky Way's satellite galaxies, Chakrabarti predicts. The two Magellanic are each about ten times larger.

If it exists, Galaxy X isn't likely to be composed entirely of dark matter.

It should also have a sprinkling of dim stars, Chakrabarti said. "These should provide enough light for astronomers to see it, now that they know where to look," she said.

The reason the dark matter galaxy hasn't yet been seen, she added, is because it lies in the same plane as the Milky Way disc. Clouds of gas and dust stand between us and Galaxy X, confounding telescopes.

(See "'Lost' Dark Matter Discovered in Space.")

Galaxy X Addresses Fundamental Problem

If Galaxy X's existence is confirmed, it would be a major step in verifying our understanding of how the universe condensed from primordial matter and energy after the big bang, Chakrabarti said.

Current theory correctly predicts the distribution of distant galaxies, she said. But it also predicts hundreds of dwarf galaxies around the Milky Way, and to date only a few dozen have been found.

This "missing satellite problem" she said, "is a fundamental problem in cosmology."

More Dark Galaxies Out There?

Galaxy X could soon lead to Galaxies Y and Z, according to Chakrabarti.

"This is basically a new method to render dark galaxies visible," she said, adding that her technique should be able to detect dim dwarf galaxies as small as a thousandth the mass of the Milky Way.

The new finding is a useful contribution to projects aiming to map the distribution of dark matter on the far edges of the universe, according to David Pooley, a Texas-based dark matter astronomer with Eureka Scientific, a private corporation that helps scientists secure research funding.

"All of these dark matter studies are really starting to map out the distribution of dark matter," said Pooley, who was not part of Chakrabarti's team. "Any information we get is extremely valuable."

(Related: "Dark Matter Helped Early Galaxies Survive 'Massacre.'")

Galaxy X: The Search Begins

Now that astronomers know where to look for Galaxy X, they should be able to find it, especially if they conduct the search in dust-penetrating infrared light, Chakrabarti said.

"Say you're looking for a car with very dim headlights, in the fog," she said. "If you know approximately where to look, you would have a better chance of finding it."

Chakrabarti hopes to do some looking herself within the next few months and is seeking to secure time at a large infrared telescope.

Even if Galaxy X isn't confirmed, she said, her findings will still shed new light on a shady subject.

The absence of X would mean there's some other oddity out there throwing off the calculations—perhaps an unexpected distribution pattern of the halo of dark matter thought to surround the Milky Way.

"We still stand to learn something very fundamental," she said.

The Galaxy X study is pending in the Astrophysical Journal.