Around our galaxy is a giant stealth. Astronomers have discovered a dwarf galaxy called Antalya 2, a third of the size of the Milky Way itself. As large as the large Magellanic cloud, the galaxy's largest companion, Antalya 2 has eluded detection until now because it is 10,000 times weaker. Such a strange animal challenges models of galaxy formation and dark matter, the invisible substance that helps pull galaxies together. The galaxy was detected by data from the European Space Agency's Gaia satellite, a space telescope that measured the movements and properties of 1 billion stars inside and around the Milky Way.