Dwarf dark galaxy hidden in ALMA gravitational lens image

Subtle distortions hidden in ALMA's stunning image of the gravitational lens SDP.81 are telltale signs that a dwarf dark galaxy is lurking in the halo of a much larger galaxy nearly 4 billion light-years away. This discovery paves the way for ALMA to find many more such objects and could help astronomers address important questions on the nature of dark matter.