Astronomers have found a galaxy that seems to be made almost entirely of dark matter.
The galaxy, known as Candidate Dark Galaxy-2 (CDG-2), is thought to be 99% made-up of the strange, invisible substance that can't be directly observed.
It took three of humanity's most powerful telescopes to find CDG-2, and astronomers say the only reason they were able to find was via the four star clusters lurking within.
More on dark matter
Finding galaxies
Most galaxies in the nearby Universe glow brightly, often with luminous centres powered by supermassive black holes at their cores.
But some galaxies are almost invisible. These dim galaxies are dominated by the mysterious substance known as dark matter.
Dark matter is a theorised substance used to explain how galaxies manage to hold themselves together.
When astronomers calculate the amount of visible matter in a galaxy – stars, gas and dust, for example – there's not enough mass to produce enough gravitational pull to hold that galaxy together.
Galaxies should be ripping themselves apart as they rapidly rotate, flinging stars out into space.
There must be some extra matter that's there, holding the galaxies together, but which is invisible. This is known as 'dark matter'.
Why CDG-2 is so dark
Astronomers used the Hubble Space Telescope, the Euclid space telescope and ground-based Subaru Telescope to locate galaxy CDG-2, but it wasn't an easy task.
It's dominated by dark matter and has a small amount of stars. It contains just four globular clusters, which are a type of ancient star cluster.
For reference, our Milky Way – a relatively small galaxy – contains over 150 globular clusters.
And astronomers say CDG-2 shines with the light of about 1 million Suns. That may sound alot, but it's incredibly dim, in cosmic terms.
CDG-2 could be among the most heavily dark matter-dominated galaxies ever discovered, say astronomers.
The team that discovered it was led by David Li of the University of Toronto, Canada.
They searched for tight groupings of globular clusters, as these can indicate the presence of a faint, hidden stellar population.
The team identified 10 previously confirmed low-surface-brightness galaxies and two additional dark galaxy candidates.
Hubble revealed a collection of four globular clusters in the Perseus galaxy cluster, which is a group of galaxies held together by gravity, 300 million lightyears from Earth.
The team then used Hubble, Euclid and Subaru for follow-up observations and found a faint glow around the star clusters. This was evidence that they were looking at a galaxy.
"This is the first galaxy detected solely through its globular cluster population," says Li.
"Under conservative assumptions, the four clusters represent the entire globular cluster population of CDG-2."
They say the globular clusters probably account for 16% of its visible content, while 99% of its mass is dark matter.
The theory is that much of the 'normal' matter that would enable stars to form in the galaxy was stripped away by gravitational interactions with other galaxies in the Perseus cluster.
"The Euclid data clearly confirm the presence of the extremely faint, diffuse light of CDG-2, revealing the galaxy behind the globular clusters for the first time," says Francine Marleau from the Institute for Astro- and Particle Physics at the University of Innsbruck in Austria.
"The Euclid images of the Perseus cluster demonstrate the mission’s unique capability to detect new low-surface-brightness galaxies, including extremely faint ones, while also revealing their globular clusters, nuclear star clusters, internal structures, and surrounding environments."
Read the full paper in the Astrophysical Journal Letters
