The night sky seems full of stars, but between them lurk clouds of gas where not a single star has ever ignited. Such objects are called dark galaxies: they contain enough gas, but it is too diffuse and cold to collapse into stars.

Astronomers ran several scenarios of the birth of our Local Group, the family of galaxies that includes the Milky Way, on supercomputers. It turned out that dark galaxies always appear in these simulations, but their number depends on how dense the gas must be to start forming stars. With a more realistic density threshold, more dark galaxies are produced.

Such a galaxy is a slowly rotating hydrogen cloud the size of a dwarf galaxy. It has no stars, so it is invisible in an ordinary telescope. But hydrogen atoms sometimes emit radio waves at a wavelength of 21 centimeters — a signal that the Chinese FAST radio telescope can detect.

According to the calculations, within 2.5 megaparsecs of us (roughly eight million light-years), there could be up to eight such objects. If they are found, it would strongly confirm the modern picture of the universe, where dark matter gathers gas into clouds but does not always make it form stars.