Astronomers have discovered an enormous "ghost" galaxy lurking near the Milk Way through the European Space Agency's Gaia satellite.
The international team, including many astronomers from the University of Cambridge, discovered the faint galaxy when examining data from Gaia's telescope.
Named Antlia 2, or Ant 2 for short, the galaxy has managed to avoid detection until now due to its extremely low density – as well as being hidden behind the shroud of the Milky Way's disk.
It is known as a dwarf galaxy, one of the first types of galaxies to emerge in the early universe.
Dwarf galaxy stars are old and quite low in mass and metal.
However compared to other dwarf galaxies orbiting the Milky Way, Ant 2 is truly enormous – about a third of the size of the Milky Way itself.
It is also eerily faint, according to normal measurements.
The University of Cambridge wrote that the galaxy is either far too large for its luminosity or far too dim for its size.
"This is a ghost of a galaxy," said Dr Gabriel Torrealba, the paper's lead author.
"Objects as diffuse as Ant 2 have simply not been seen before. Our discovery was only possible thanks to the quality of the Gaia data."
Co-author Dr Sergey Koposov, from Carnegie Mellon University, added: "The simplest explanation of why Ant 2 appears to have so little mass today is that it is being taken apart by the Galactic tides of the Milky Way.
"What remains unexplained, however, is the object's giant size. Normally, as galaxies lose mass to the Milky Way's tides, they shrink, not grow."
According to the University of Cambridge, if it is impossible for the dwarf galaxy to have grown as the Milky Way stripped its mass away, then Ant 2 must have been truly enormous.
Alternatively, the academics suggest that Ant 2's low density could mean that a modification to the properties of dark matter is needed.
The current favoured theory predicts dark matter to pack tightly in the centres of galaxies.
But given the lack of density within Ant 2, it suggests another dark matter particle which does not cluster may be needed.
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"Compared to the rest of the 60 or so Milky Way satellites, Ant 2 is an oddball," said co-author Dr Matthew Walker, from Carnegie Mellon University.
"We are wondering whether this galaxy is just the tip of an iceberg, and the Milky Way is surrounded by a large population of nearly invisible dwarfs similar to this one."
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