Astronomers have discovered a vast, glowing blob of gas in the distant Universe, and they can't figure out what's actually lighting it up.
This glowing nebula is located at the centre of an enormous 'protocluster' of early galaxies some 10 billion light-years from Earth, and is the brightest cosmic object of its kind ever found. And yet, there's no obvious source of its power.
The object, called an 'enormous Lyman-alpha nebula' (ELAN), is not only the brightest object of its kind found in the Universe - it's also one of the biggest, rivalling even the 'Slug Nebula', which stretches 2 million light-years through intergalactic space.
"It's extremely bright, and it's probably larger than the Slug Nebula, but there's nothing else visible except the faint smudge of a galaxy," says one of team, J. Xavier Prochaska, from the University of California, Santa Cruz.
"So it's a terrifically energetic phenomenon without an obvious power source."
Only a handful of ELANs have been discovered so far, and these accumulations of gas are thought to be part of a network of filaments that connect neighbouring galaxies to each other in the vast cosmic web of the Universe.
While other ELANs appear to be powered by the intense radiation given off by quasars, star formation, or supernovae, no such events could be found near this latest example, dubbed the MAMMOTH-1 nebula.
What we do know is the light has the same wavelength that's absorbed and emitted by hydrogen atoms as they cool down - a discharge known as Lyman-alpha radiation - but it's not clear what's been heating them up.
Scientists have suggested that supermassive black holes swallowing matter in galaxies within the central region of the blob could be responsible.
The MAMMOTH-1 nebula was found by a survey called Mapping the Most Massive Overdensities Through Hydrogen (MAMMOTH), which was also responsible for finding the Slug Nebula back in 2014.
The protocluster it's found in is massive, hosting an unusually high concentration of galaxies in an area of about 50 million light-years across - all bound together by gravity.
While the galaxies are now mature, our telescopes are only just seeing them as they would have looked a mere 3 billion years after the Big Bang - the peak of galaxy formation in our Universe.
Several hypotheses have been put forward for how the MAMMOTH-1 nebula at the heart of this great protocluster got so bright, but the most likely ones revolve around radiation or discharges coming from something called an active galactic nucleus (AGN).
AGNs are compact regions at the centre of galaxies that have a much higher than normal luminosity. The team explains that AGNs are powered by a supermassive black hole actively feeding on gas in the centre of a galaxy, and are known for being extremely bright sources of light.
The intense radiation emitted by AGNs ionise the hydrogen gas in the space around it, and this could prompt the ELANs to emit super-bright Lyman-alpha radiation.
Quasars - the brightest objects in the Universe - are known for being the most luminous AGNs in visible light, except the MAMMOTH-1 nebula is not associated with a quasar, as far as the researchers can tell.
But that doesn't mean there isn't one lurking in the background.
"It has all the hallmarks of an AGN, but we don't see anything in our optical images. I expect there's a quasar that is so obscured by dust that most of its light is hidden," Prochaska says.
The team has its work cut out for it in trying to spot a single quasar some 10 billion light-years from Earth, and until then, the brightest known ELAN will remain a cosmic mystery.
The research has been accepted for publication in the Astrophysical Journal, and you can read it in full at the pre-print website, arXiv.org.