At $32.3\,{\rm mag}\,{\rm arcsec}^{-2}$, the Antlia II dwarf galaxy is the most diffuse galaxy ever observed in terms of surface brightness. It was only discovered by pre-selecting stars based on their proper motions as measured by the Gaia mission:

Fig. 1 from the article linked above.

The discovery article includes a discussion of possible origins, but ultimately states caveats to all proposed scenarios. How could such an extreme galaxy come to exist?


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