The vast gassy limb comprises an arc of hydrogen 77,000 light years long and several thousand light years thick, running along the Milky Way's outermost edge and sweeping around the four main arms that swirl out from the galaxy's core.
As it is not in the visible part of the light spectrum, it cannot be seen by telescope.
Astronomers at the Australia National Telescope Facility in Epping, New South Wales, made the discovery in a project to map the distribution of hydrogen gas across the galaxy.
Most of the Milky Way is obscured by interstellar dust, but hydrogen emits radio waves which pass through the dust clouds and which thus make it detectable by radio telescope.
"We see it [the arm] over a huge area of sky," lead astronomer Naomi McClure-Griffiths said.
She speculates the arm is a long gaseous tendril that was once joined up with another spiral limb but became detached.
The study will be published in Astrophysical Journal Letters.
Astronomers are amazed that the feature had been overlooked, New Scientist says.
"I was absolutely flabbergasted, it was quite clearly seen in some of the previous surveys but it was never pointed out or given a name," said Tom Dame at the Harvard-Smithsonian Center for Astrophysics in Massachusetts.