NASA’s JWST Hunts Dark Matter in Gorgeous Picture of Bullet Cluster

For astronomers finding out dark matter, the Bullet Cluster is likely one of the best laboratories within the universe.

It was found nearly by chance, a blip of x-rays in the sky that was detected by NASA’s Einstein Observatory in 1992 and given the designation 1E 0657-56. Follow-up observations in seen gentle confirmed it to be a galaxy cluster—a swarm of dozens and even a whole bunch of galaxies all sure collectively by gravity and orbiting a typical middle. The 1E 0657-56 cluster is decently far-off from Earth; the sunshine we see left it about 4 billion years in the past.

The cluster isn’t just one easy system of galaxies, although. There’s a primary cluster, large and considerably elongated, with a extra compact and spherical subcluster offset to at least one facet, separated by greater than 1.5 million light-years. Note that the closest large galaxy to our personal Milky Way is the Andromeda galaxy, 2.5 million light-years away. Imagine having dozens of galaxies in our sky at lower than half that distance!

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Deeper observations of 1E 0657-56 taken using the Chandra X-ray Observatory revealed the cluster was loaded with scorching gasoline, and I do imply scorching: most of it was tens of thousands and thousands of levels Celsius. This is frequent in galaxy clusters, the place such gasoline is mostly assumed to be superheated by radiation spewed out from supermassive black holes, in addition to the large quantity of kinetic power absorbed as galaxies zoom round within the cluster.

But the gasoline in 1E 0657-56 had an odd construction. Like the cluster itself, it was divided into two primary clouds, each positioned in between the primary cluster and the subcluster. The greater cloud, nearer the primary cluster, was elongated and diffuse. But the opposite one, nearer to the compact subcluster, was smaller and had a attribute bow shock form, a uninteresting cone just like the wake left behind as a ship strikes by water.

This meant 1E 0657-56 wasn’t a single cluster however really two clusters that lately collided—“recent” on a cosmic scale, that’s: about 200 million to 100 million years in the past. The collision occurred at breathtakingly excessive pace, with the 2 clusters slamming into one another at a relative velocity of about 4,000 kilometers per second. That’s greater than 1 % of the pace of sunshine!

The conical form of the gasoline gave the system its nickname of the Bullet Cluster, which can be acceptable as a result of the second cluster is smaller than the opposite one and seems to have blasted proper by it.

Galaxies are small in contrast with the dimensions of the Bullet Cluster, so only a few galaxies in it bodily collided. In a way, this cosmic object’s two smaller constituent clusters handed proper by one another. But the new gasoline that suffused the house between the galaxies in every cluster would have crashed head-on. While the galaxies slipped by comparatively unscathed, that scorching gasoline was slowed significantly by the collision. That’s why a lot of the gasoline is positioned between the 2 galaxy clusters; it was left behind on the scene of the smashup between the 2 fleeing methods.

But there’s extra to the Bullet Cluster than meets the attention.

For many years, astronomers have amassed numerous proof for the existence of darkish matter—a mysterious substance that has mass and gravity however emits no gentle and infrequently, if ever, interacts with regular matter.

On cosmic scales, darkish matter betrays its presence through its gravity. The pace at which stars orbit round in a galaxy is dependent upon the gravity they really feel from the galaxy at massive, which in flip is dependent upon how a lot mass it has—that’s, how a lot matter it holds. The extra mass, the stronger the gravity, and the quicker a star strikes. American astronomers Vera Rubin and Kent Ford used this precept within the Seventies to point out that stars within the outermost a part of the Andromeda galaxy had been shifting far too quickly, given Andromeda’s measured mass. This implied there was a halo of darkish matter by which the galaxy was embedded.

Something related has been seen in lots of galaxy clusters: the galaxies are shifting far too shortly for the calculated mass of their house cluster. They ought to fly off into house, however as a substitute they keep in orbit, implying there’s much more mass to those clusters that we can’t see.

Whatever darkish matter is, it isn’t thought to work together with regular matter besides by gravity, and it’s also predicted to not work together properly even with itself. That means in case you have two colliding objects surrounded by darkish matter, these halos will cross proper by one another and proceed into house.

You most likely see the place that is going: the Bullet Cluster is precisely that form of scenario, a dark matter experiment simply ready for us to look at. Detecting the darkish matter, although, requires a gravitational trick.

When a beam of sunshine passes by an object with mass, the gravity of that object will bend the trail of the sunshine ray. For very large or dense objects, the sunshine can bend considerably. For instance, gentle from a background galaxy will be warped into an arc form, or be damaged up into a number of photographs. This phenomenon is known as strong gravitational lensing as a result of it acts very very like a glass lens.

If the gravity of a lens isn’t as sturdy, it will probably nonetheless mildly distort the picture of a background galaxy, however it’s onerous to know the way distorted any particular person galaxy may be. This form of weak gravitational lensing will be detected statistically, nonetheless, by taking a look at a large number of background galaxies and measuring their shapes.

Astronomers mapped the weakly lensed galaxies seen behind the Bullet Cluster, which they then used to hint the place of the cluster’s darkish matter. What they discovered was superb: there was an enormous extra of mass surrounding each subclusters! That meant the darkish matter halos of the subclusters handed proper by one another, simply as idea predicted.

Because of this, the Bullet Cluster is taken into account by almost all astronomers because the smoking gun (pun very a lot meant) for darkish matter’s existence, particularly in halos surrounding galaxies and clusters.

But scientists aren’t accomplished inspecting the cluster. An international team of astronomers observed it with the James Webb Space Telescope (JWST), which allowed them to see many extra distant background galaxies, which in flip allow them to map the darkish matter utilizing gravitational lensing in way more element. They published their results inthe Astrophysical Journal Letters in June 2025.

JWST’s subject of view is considerably small, in order that they didn’t observe your entire cluster, however they had been nonetheless in a position to gauge its mass and discover that the entire cluster—stars, scorching gasoline, darkish matter and all—comprises a number of hundred trillion occasions the mass of the solar. That’s really smaller than earlier estimates, which can be linked partly to JWST’s smaller subject of view however might also be an actual outcome primarily based on its sharper imaginative and prescient. The group is at present engaged on analyzing knowledge from each JWST and the large Dark Energy Camera to see if they will refine the mass estimate.

The researchers additionally be aware that the JWST knowledge present that the elongated primary cluster comprises at the least three clumps of galaxies, whereas a smoother distribution is anticipated. This means the primary cluster could have undergone different collisions lately, additional complicating the Bullet Cluster’s already advanced historical past.

Invisible darkish matter could also be, however that doesn’t imply undetectable. And each time we level a brand new telescope on the Bullet Cluster, we be taught extra about it. We’re closing in on darkish matter, and shortly, hopefully, its constituents will probably be illuminated.