Astronomers have long suspected strange ripples in hydrogen gas in the disk of our Milky Way galaxy are caused by the gravity of an unseen dwarf galaxy dominated by dark matter -- and now they think they've found this "Galaxy X."

The prediction of an invisible dark matter dwarf galaxy orbiting our Milky Way, made in 2009, may have had its "observational confirmation," say researchers at the Rochester Institute of Technology in New York.

RIT astronomer Sukanya Chakrabarti and her colleagues analyzed near-infrared data from the European Southern Observatory's VISTA telescope to discover four young stars clustered in the constellation Norma.

The stars are 300,000 light-years distant, well beyond the edge of the Milky Way's disk, they report in the Astrophysical Journal Letters.

"They can't be part of our galaxy because the disk of the Milky Way terminates at 48,000 light years," says Chakrabarti.

"These young stars are likely the signature of this predicted galaxy," she says, suggesting the dwarf galaxy is difficult to see through the obscuring dust of our own galaxy and because the majority of its mass is invisible dark matter.

Chakrabarti was the first to predict its existence in 2009 based on her analysis of the ripples seen in our home galaxy's outer disk.

"I decided to see if I could actually find the thing," she says. "It was a difficult prediction to test because it was close to the plane [of our Milky Way galaxy], and therefore difficult to see in the optical."

The infrared capability of the VISTA telescope allowed Chakrabarti and her fellow astronomers to peer into previously unexplored regions close to our galaxy's disk plane that are inaccessible using visible light.

Dark matter is a hypothetical but never directly observed form of matter believed to account for most of the matter and mass of the universe.

Its existence has been inferred from effects observed on visible matter and on the structure of the universe, attributed to dark matter's gravitational pull.

Large galaxies like our own Milky Way likely have many small satellite galaxies dominated by dark matter that are thus difficult to see, astronomers have suggested.

Chakrabarti's 2009 study had predicted a specified mass and location for the unseen galaxy, and radiation from the four news discovered stars, known as Cepheid variables, allowed her to derive accurate distances and test her prediction.

"The discovery of the Cepheid variables shows that our method of finding the location of dark-matter dominated dwarf galaxies works," she says.

"It may help us ultimately understand what dark matter is made up of," she adds.

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