A group of scientists may have solved one of the biggest mysteries in the Universe, following the discovery of a vast, glowing thread of gas bridging four galaxy clusters in deep space.
Stretching 23 million lightyears and burning at over 10 million°C, the filament could provide the answer to an enduring question in astronomy: where is Universe’s missing matter?
More amazing space science
Despite all the stars, planets and galaxies that we can directly observe, astronomers say over a third of the Universe’s normal matter — the 'stuff' that makes up everything we see and know — is missing.
This isn't a reference to dark matter, or dark energy, which are inferred but can't be seen.
The missing matter is part of the stuff we can see, and the problem has been puzzling scientists for decades.
Discovering the missing matter
Scientists made the discovery using two powerful space telescopes: the European Space Agency’s XMM-Newton and Japan’s Suzaku.
Models of the Universe predict this ordinary matter should be out there, strung across space in long strings of gas – filaments – bridging gaps between the densest pockets of matter in the Universe.
These cosmic bridges have been seen before, but astronomers say they're faint, making it difficult to see them among the glare of bright objects like galaxies, black holes and other nearby objects.
But this new study is one of the first to do this, finding and characterising a single filament of hot gas stretching between four clusters of galaxies in the nearby Universe.
"For the first time, our results closely match what we see in our leading model of the cosmos – something that’s not happened before," says lead researcher Konstantinos Migkas of Leiden Observatory in the Netherlands.
"It seems that the simulations were right all along."
A thread through the cosmos
The filament is 10 times as massive as our Milky Way galaxy and connects four galaxy clusters – two on one end, two on the other – all part of the Shapley Supercluster, a group of over 8,000 galaxies and one of the biggest structures in the nearby Universe.
XMM-Newton allowed scientists to pinpoint and remove bright sources of X-ray — like those from distant supermassive black holes — that would otherwise have muddied the view.
"Thanks to XMM-Newton we could identify and remove these cosmic contaminants, so we knew we were looking at the gas in the filament and nothing else," says co-author Florian Pacaud of the University of Bonn, Germany.
"Our approach was really successful, and reveals that the filament is exactly as we’d expect from our best large-scale simulations of the Universe."
Suzaku provided a broad map of the X-ray glow emitted by the hot gas, showing just how vast and dense the filament really is.
Uncovering the foundations of the Universe
This filament is a key piece of evidence supporting how scientists think the Universe is built.
It shows how some of the densest, most massive structures in the Universe – galaxy clusters – are connected over huge distances across the cosmos.
And the fact the discovery lines up with theoretical models of cosmic evolution enables scientists to further assume their models are correct.
It's also greater insight into the nature of the 'cosmic web’ the vast, invisible cobweb of filaments that underpins the structure of the observable Universe.
"This research is a great example of collaboration between telescopes, and creates a new benchmark for how to spot the light coming from the faint filaments of the cosmic web," says Norbert Schartel, ESA XMM-Newton Project Scientist.
"More fundamentally, it reinforces our standard model of the cosmos and validates decades of simulations.
"It seems that the ‘missing’ matter may truly be lurking in hard-to-see threads woven across the Universe."
Euclid to follow
The European Space Agency's Euclid mission, which has already begun releasing its first images and science data, is tasked with investigating dark matter and dark energy, which make up 95% of the Universe, but have never directly been observed.
Euclid is also tasked with uncovering more information about the cosmic web, so this latest discovery is a step in the right direction.
So where is the Universe’s missing matter?
It may, it seems, be glowing faintly in the filaments of the cosmic web, stretched across space like strands of an invisible spider’s silk.
And this latest discovery is one more step along the road to understand exactly the foundations of the cosmos.
