Invisible Galaxies: 70 Candidates for Dark Worlds express
The FAST radio telescope in China is a giant dish half a kilometer in diameter, nestled in a natural sinkhole among mountains. It catches radio waves from space, and in a fresh sky survey it picked up 70 strange objects: they emit a radio signal characteristic of hydrogen clouds, but through an ordinary optical telescope almost nothing is visible at their location. Such objects are called dark galaxy candidates.
A dark galaxy is a gas cloud, mostly hydrogen, too diffuse for stars to ignite. Stars are born when gas collapses under its own gravity, but if the cloud is too fluffy or something heats it up, it remains dark. Hydrogen, however, can faintly glow in radio at a wavelength of 21 centimeters — a kind of atomic radio whisper. FAST can hear this whisper from distances up to 50 megaparsecs (about 163 million light-years).
The authors compared the found dark galaxies with normal, star-bearing ones. It turned out that at the same hydrogen mass, dark galaxies rotate faster: their radio lines are broader. This hints that they contain more dark matter, which keeps the gas spinning faster. In the nearby vicinity up to 11 megaparsecs, no such objects were found at all: perhaps near us they have already been 'eaten' by larger neighbors.
The catalog of 70 candidates is a first step. Next, astronomers will point more sensitive optical telescopes at them to definitively confirm: these are truly galaxies without stars, not measurement errors. If confirmed, it will change our understanding of how galaxies are born and die.
The FAST radio telescope is so sensitive that it could detect a cell phone signal on the Moon, if one were there.
In 'Star Trek', dark galaxies sometimes serve as hiding places for ships because they are hard to spot from a distance.