Astronomers have discovered an enormous "ghost" galaxy orbiting around the Milky Way.
The very faint dwarf galaxy called Antlia 2, is about a third the size of our home galaxy and appears to have very few stars.
Antlia 2 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.
This low mass is believed to be part of the reason why the galaxy has managed to avoid detection until now. It has also been hidden behind the shroud of the Milky Way's disk.
"This is a ghost of a galaxy," said Gabriel Torrealba, lead author of the paper describing the discovery, which was published on November 9.
"Objects as diffuse as Antlia 2 have simply not been seen before.
"Our discovery was only possible thanks to the quality of the Gaia data."
The new discovery now challenges conventional theories on how galaxies are formed as scientists cannot fully explain how a galaxy so large has so few stars.
Co-author Dr Alex Geringer-Sameth, from the Department of Physics at Imperial College London, said: "Antlia 2 might be hinting towards some new fundamental forces at work — in particular that dark matter might not behave as simply as has been assumed in the standard model of cosmology."
It has also prompted astronomers to speculate about whether vast numbers of galaxies are hiding just outside our own.
Dr Matthew Walker, from Carnegie Mellon University, added: "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."