Astronomers have discovered the largest galaxy ever discovered, and they have no idea how it became that massive.

The Alcyoneus galaxy, which spans 16.3 million light-years, is 160 times larger than the Milky Way and four times larger than the previous record-holder, IC 1101, which covers 3.9 million light-years, according to a recent study. Alcyoneus is approximately 3 billion light-years from Earth and is named after one of the fabled giants who fought Hercules and whose name means "mighty ass" in Greek.

The galactic monster is a very huge example of a radio galaxy or a galaxy with a supermassive black hole at its core that gobbles up enormous amounts of the matter before spewing it out in the form of two immense jets of plasma flying at close to the speed of light. The plasma beams slow down after travelling millions of light-years, spreading out into plumes that release light in the form of radio waves. The lobes of Alcyoneus are the largest ever discovered.

Galaxies with massive, plasma-filled radio lobes aren't uncommon (the Milky Way has two modest plumes), but astronomers are baffled as to how Alcyoneus, a pretty ordinary galaxy at its core, was able to generate such monstrously large lobes. The researchers published their findings on the preprint service arXiv on Feb. 11, which have been approved for publication in the journal Astronomy & Astrophysics.

"We discovered what is expected to be the largest known structure formed by a single galaxy — a massive radio galaxy with a proper length of 4.99 0.04 megaparsecs [16.28 million light-years]. 5.04 0.05 megaparsecs [16.44 million light-years] is the true proper length "The study was written by the researchers, who were directed by Martijn Oei, an astronomer at the Leiden Observatory in the Netherlands.

The new galactic heavyweight was discovered by the researchers while combing over data collected by the Low-Frequency Array (LOFAR), a network made up of around 20,000 radio telescopes spread across 52 locations in Europe. Oei discovered the massive structure by chance after processing the data to find only huge and diffuse radio lobes.

Apart from its immense jets, Alcyoneus is a typical elliptical galaxy, with a total mass of 240 billion times the mass of the sun (half that of the Milky Way) and a central supermassive black hole 400 million times the mass of the sun (100 times less massive than the largest black hole). Alcyoneus' centre is modest in comparison to the centres of most radio galaxies.

And it wasn't simply Alcyoneus' mass that seemed out of place.

"Aside from geometry, Alcyoneus and its host [galactic centre] are suspiciously ordinary: the total low-frequency luminosity density, stellar mass, and supermassive black hole mass are all lower than, though comparable to, those of the medial giant radio galaxies," the researchers wrote in their study. "Thus, very massive galaxies or central black holes are not required to generate huge giants, nor is strong radio power if the observed condition is indicative of the source across its lifetime."

For the time being, the astronomers are perplexed, but they are looking into several possible answers. One theory is that the galaxy's surroundings have a lower density than typical, allowing its jets to spread to unprecedented scales. Another possibility is that Alcyoneus lives within a filament of the cosmic web, which is a massive and poorly understood structure of gas and dark matter that connects galaxies.

The researchers believe that determining what causes Alcyoneus to increase in size would help them understand how other galaxies grow as well. "If there are host galaxy properties that are a significant source of giant radio galaxy expansion," the researchers said, "then the hosts of the greatest giant radio galaxies are likely to contain them." "Similarly, if there are specific large-scale environments that are particularly favourable to the evolution of gigantic radio galaxies, the largest giant radio galaxies are likely to live in them."