If a remote galaxy is located exactly behind another galaxy then it can be seen deformed by gravitational lensing as an "Einstein ring". The careful analysis of such an effect has recently revealed the presence of an invisible dwarf galaxy contributing to the gravitational lens. It could be one of the many dark-matter satellites expected by simulations of cold dark matter.

Playing hide-and-seek is not easy for galaxies: they appear even bigger and brighter when they hide behind each other. When two galaxies are perfectly aligned on the line-of-sight, the image of the remoter one can even be deformed into a complete annulus around the nearer source. This strong gravitational lensing effect is called an "Einstein ring" because it is described by Albert Einstein’s general theory of relativity. The gravity of the closer galaxy acts like a magnifying glass. It bends the light paths around it by deforming space–time locally.

According to standard cosmology, the universe is composed of about 5% ordinary matter (baryons), 23% dark matter and 72% dark energy. This result is derived from fluctuations of the cosmic microwave background (CMB) with additional constraints from type Ia supernovae and from the large-scale distribution of galaxies (CERN Courier May 2008 p8). Because dark-matter particles are still of unknown nature, they are distinguished as "cold" or "hot", where "hot" means that they are moving with a highly relativistic speed, like neutrinos, for instance.

If dark matter is cold, small-scale galaxies should form first and subsequently merge to form larger galaxies; if it is hot, large halos would form first and then fragment into galaxies of various sizes. Numerical simulations of the formation of large-scale structures from CMB fluctuations suggest that dark matter should be dominated by a cold component to account for the observed distribution of galaxies. There are, however, several problems with cold dark matter. One of them is that such simulations greatly over-predict the number of small galaxies orbiting fully fledged spiral or elliptical galaxies. While cold dark matter should result in thousands of dwarf satellite galaxies around the Milky Way, astronomers have so far observed only about 30 of them. So either the models are missing a key ingredient or there should be many dark-matter galaxies around the Milky Way – but too scarcely populated with stars to be detectable.

While some astronomers search for dark galaxies around the Milky Way, others try to detect their presence around distant galaxies using gravitational lensing. Simona Vegetti, a postdoctoral researcher at the Massachusetts Institute of Technology, belongs to the second group and was lucky enough to detect the signature of one such elusive galaxy at cosmological distance. Together with her colleagues in the Netherlands and US, she studied the B1938+666 lens system, in which the gravitational lensing effect of a massive elliptical galaxy at a redshift of z≃0.9 is making a background galaxy (z≃2.1) appear as an almost perfect Einstein ring. The study is based on infrared observations obtained both with the Keck 10 m telescope on Mauna Kea, Hawaii, and with the Hubble Space Telescope.

The team found a slight deformation of the Einstein ring that would be the imprint, according to their modelling, of a small companion to the lensing elliptical galaxy. The dwarf galaxy’s weight was estimated at 190 million solar masses and its luminosity has an upper limit of 54 million solar luminosities. This means that its mass-to-light ratio is at least 3.5 times higher than that of the Sun. Such a ratio is typical of dwarf galaxies orbiting the Milky Way, such as Fornax and Sagittarius, but it is still an order of magnitude below the expectation of numerical simulations. As long as the Galaxy remains invisible even to future facilities, the discovery of this object will support the cold dark-matter scenario, although many more such small dark galaxies must be discovered to be consistent with simulations.