A massive glowing cloud of gas has been spotted 10 billion lightyears away in the distant Universe, but without an obvious source for the light it is giving off.
The object is known as an enormous Lyman-alpha nebula (ELAN), which are huge clouds of gas that exist between galaxies.
They are thought to be part of a network of filaments that connect galaxies known as the cosmic web, most of which is thought to be made up of invisible dark matter.
Previously discovered ELANs are theorised to be illuminated by radiation from bright objects known as quasars, but this latest discovery, named MAMMOTH-1, appears to have no obvious energy source.
It was discovered in the middle of a region with a high concentration of early galaxies, known as a protocluster.
These protoclusters are the precursors to galaxy clusters, which are massive groups of galaxies held together by gravity.
Because MAMMOTH-1 is so far away, when astronomers observe it they are effectively looking back in time to how the cluster appeared 10 billion years ago, about 3 billion years after the Big Bang.
Today, it would probably be a fully formed galaxy cluster.
The ELAN appears to have a filamentary structure aligned with galaxy distribution in the protocluster,which supports a theory that says ELANs are illuminated segments of the cosmic web.
As for its mysterious source power, the energy could be radiation from an active galactic nucleus – a bright galactic core – obscured by space dust.
“Our survey was not trying to find nebulae.
We’re looking for the most overdense environments in the early universe, the big cities where there are lots of galaxies,” says Zheng Cai, a Hubble Postdoctoral Fellow at UC Santa Cruz.
“We found this enormous nebula in the middle of the protocluster, near the peak density.”