For astronomers finding out darkish matter, the Bullet Cluster is without doubt one of the biggest laboratories within the universe.
It was found virtually accidentally, a blip of x-rays within the sky that was detected by NASA’s Einstein Observatory in 1992 and given the designation 1E 0657-56. Observe-up observations in seen gentle confirmed it to be a galaxy cluster—a swarm of dozens and even tons of of galaxies all sure collectively by gravity and orbiting a typical heart. The 1E 0657-56 cluster is decently distant from Earth; the sunshine we see left it about 4 billion years in the past.
The cluster is not only one easy system of galaxies, although. There’s a important cluster, massive and considerably elongated, with a extra compact and spherical subcluster offset to 1 facet, separated by greater than 1.5 million light-years. Observe that the closest massive galaxy to our personal Milky Manner is the Andromeda galaxy, 2.5 million light-years away. Think about having dozens of galaxies in our sky at lower than half that distance!
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Deeper observations of 1E 0657-56 taken utilizing the Chandra X-ray Observatory revealed the cluster was loaded with sizzling fuel, and I do imply sizzling: most of it was tens of hundreds of thousands of levels Celsius. That is frequent in galaxy clusters, the place such fuel is usually assumed to be superheated by radiation spewed out from supermassive black holes, in addition to the large quantity of kinetic vitality absorbed as galaxies zoom round within the cluster.
However the fuel in 1E 0657-56 had an odd construction. Just like the cluster itself, it was divided into two important clouds, each situated in between the primary cluster and the subcluster. The larger cloud, nearer the primary cluster, was elongated and diffuse. However the different one, nearer to the compact subcluster, was smaller and had a attribute bow shock form, a boring cone just like the wake left behind as a ship strikes via water.
This meant 1E 0657-56 wasn’t a single cluster however really two clusters that not too long ago collided—“current” 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 fuel gave the system its nickname of the Bullet Cluster, which can also be acceptable as a result of the second cluster is smaller than the opposite one and seems to have blasted proper via it.
Galaxies are small in contrast with the scale 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 via one another. However the sizzling fuel that suffused the house between the galaxies in every cluster would have crashed head-on. Whereas the galaxies slipped by comparatively unscathed, that sizzling fuel was slowed significantly by the collision. That’s why a lot of the fuel is situated between the 2 galaxy clusters; it was left behind on the scene of the smashup between the 2 fleeing methods.
However there’s extra to the Bullet Cluster than meets the attention.
For many years, astronomers have amassed a number of proof for the existence of darkish matter—a mysterious substance that has mass and gravity however emits no gentle and barely, if ever, interacts with regular matter.
On cosmic scales, darkish matter betrays its presence by way of its gravity. The pace at which stars orbit round in a galaxy is dependent upon the gravity they really feel from the galaxy at giant, 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 sooner a star strikes. American astronomers Vera Rubin and Kent Ford used this precept within the Nineteen Seventies to point out that stars within the outermost a part of the Andromeda galaxy have been shifting far too quickly, given Andromeda’s measured mass. This implied there was a halo of darkish matter through which the galaxy was embedded.
One thing related has been seen in lots of galaxy clusters: the galaxies are shifting far too shortly for the calculated mass of their dwelling cluster. They need to fly off into house, however as an alternative they keep in orbit, implying there’s much more mass to those clusters that we can not see.
No matter darkish matter is, it isn’t thought to work together with regular matter besides via gravity, and it’s also predicted to not work together properly even with itself. Meaning if in case you have two colliding objects surrounded by darkish matter, these halos will go proper via one another and proceed into house.
You most likely see the place that is going: the Bullet Cluster is precisely that kind of scenario, a darkish 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 huge or dense objects, the sunshine can bend considerably. For instance, gentle from a background galaxy could be warped into an arc form, or be damaged up into a number of pictures. This phenomenon known as robust gravitational lensing as a result of it acts very very similar to a glass lens.
If the gravity of a lens isn’t as robust, it will probably nonetheless mildly distort the picture of a background galaxy, however it’s arduous to understand how distorted any particular person galaxy is likely to be. This kind of weak gravitational lensing could 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 wonderful: there was an enormous extra of mass surrounding each subclusters! That meant the darkish matter halos of the subclusters handed proper via one another, simply as principle predicted.
Due to this, the Bullet Cluster is taken into account by practically all astronomers because the smoking gun (pun very a lot meant) for darkish matter’s existence, particularly in halos surrounding galaxies and clusters.
However scientists aren’t finished inspecting the cluster. A world group of astronomers noticed it with the James Webb Area 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 much more element. They revealed their outcomes inthe Astrophysical Journal Letters in June 2025.
JWST’s discipline of view is considerably small, so that they didn’t observe your entire cluster, however they have been nonetheless capable of gauge its mass and discover that the entire cluster—stars, sizzling fuel, darkish matter and all—comprises a number of hundred trillion instances the mass of the solar. That’s really smaller than earlier estimates, which can be linked partly to JWST’s smaller discipline of view however may be an actual consequence primarily based on its sharper imaginative and prescient. The group is at the moment engaged on analyzing knowledge from each JWST and the large Darkish Power Digital camera to see if they’ll refine the mass estimate.
The researchers additionally notice that the JWST knowledge present that the elongated important cluster comprises at the least three clumps of galaxies, whereas a smoother distribution is anticipated. This implies the primary cluster could have undergone different collisions not too long ago, additional complicating the Bullet Cluster’s already complicated 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 study extra about it. We’re closing in on darkish matter, and shortly, hopefully, its constituents shall be illuminated.
