Astronomers have found an elusive type of miniature galaxy.

Dwarf galaxies formed out of dark matter in the early Universe, but only a small number have been detected. Yashar Hezaveh of Stanford University in California and his colleagues studied images taken by the high-resolution Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. They found a galaxy that was acting like a lens, gravitationally bending light from another, more distant galaxy to form a ring of mirages in the images. The team spotted an additional 'smudge' on these mirages caused by an otherwise invisible dwarf galaxy orbiting the lensing galaxy.

The authors say that ALMA should be able to uncover more dark dwarf galaxies, which would bolster existing models of dark matter.

Astrophys. J. in the press; preprint at http://arxiv.org/abs/1601.01388 (2016)