Space can be a violent place. Asteroids and comets slam into planets, stars explode – or they are ripped apart by black holes.
But in terms of scale, perhaps nothing is as violent as the collisions between huge clusters of galaxies.
These cosmic conglomerations typically contain hundreds of galaxies – each boasting billions to trillions of stars – bound together by gravity. When one cluster gets too close to another, their mutual gravity yanks them together. The resulting collisions are huge, generating more energy than any other event since the Big Bang.
One of the biggest and most famous is the Bullet Cluster, which involves two clusters – and more than a thousand galaxies in total – in mid-crash. The impact is so strong that it created a huge, bullet-shaped shockwave, earning its name.
Sure, the wreckage is impressive. But astronomers aren't just rubberneckers leering at spectacle. What's caught their attention is that the collision has uncovered the invisible: dark matter.
Dark matter is thought to be the glue that binds the clusters together, forming the gravitational foundation of stars, gas, and galaxies throughout the Universe. It comprises a quarter of the cosmos, yet it's not visible and no one knows what it is.
But thanks to the Bullet Cluster, scientists can enjoy a front-row view of the mysterious stuff. The collision has exposed the dark matter, separating it from the regular matter of stars and gas – producing one of the best pieces of direct evidence that dark matter really exists.
"It is my favourite object in the Universe," says Marusa Bradac, an astrophysicist at the University of California, Davis, in the US.
And that's not just because of what it's telling scientists about dark matter. The cluster serves as a versatile tool, helping researchers study some of the first galaxies that ever existed. It's also a cosmic laboratory where physicists can study the fundamental behaviour of hot, electrically charged gas called plasma.
The Bullet Cluster lies in the southern sky, 3.7 billion light-years away from Earth. It consists of two separate clusters slamming into each other at speeds of roughly 6 million mph (10 million km/h). While fast by human standards, it's not going to break any cosmic records (some stars can go way faster). But what makes the collision so energetic is its sheer size.
One cluster is a thousand trillion times more massive than the sun. The other weighs about a hundred trillion suns. When you have such heavyweights smashing into each other, their constituent gases heat up to 200 million C, making the Bullet Cluster one of the hottest clusters known.
We see it as if we are spectators sitting exactly to the side of it
Researchers estimate that the total energy of the collision is roughly 1 octodecillionJoules – which, to state the obvious, is a ridiculous amount of energy. It's equivalent to the energy produced by about six trillion suns burning for as long as the age of the Universe, about 13.8 billion years.
The whole crash is vast in both time and space. It would take a light beam 6 million years to travel from one end to the other. The collision as seen today has been going on for 150 million years, which is a very short time in cosmic terms. This gives astronomers an unprecedented glimpse of a crash just as it's happening. It won't all settle down for a couple billion more years.
While the Bullet's timing, and prodigious mass and energy certainly set it apart, it is its orientation that makes it even more unique. By coincidence, the collision is aligned perpendicular to our line of sight. "We see it as if we are spectators sitting exactly to the side of it," says Andrey Kravtsov, an astrophysicist at the University of Chicago in Illinois, US.
With the best seats in the house, astronomers can examine the collision with more precision than any other cluster so far. Which has allowed them to directly detect dark matter for the first time.
In 2006, following up on some tantalising observations two years prior, Bradac and a team of astronomers led by Douglas Clowe, now at Ohio University in the US, analysed new data from the Hubble Space Telescope and the Chandra X-ray Observatory, a space telescope that sees in X-rays – crucial for mapping where the normal matter is in the cluster.
The vast majority of normal matter in a cluster consists of gas. Because the cluster is so massive, the gravity tugs strongly on the gas molecules, speeding them up and getting them hot – which causes them to emit X-ray radiation. Colliding clusters just heat up the gas further, generating more energetic X-rays. This means that the X-rays can reveal to astronomers where most of the normal matter in the cluster lies.
For the first time, astronomers detected dark matter in naked isolation
Then, using the Hubble data, the researchers could map the location of the clusters' mass. Anything with mass – both normal and dark matter – has gravity that can bend the light from galaxies in the background. The more mass, the stronger it acts as a lens, and the more the light gets warped.
Putting these observations together, the astronomers discovered that most of the mass was not where most of the gas was. About 80% of the Bullet's mass was concentrated in a region that wasn't glowing the strongest in X-rays or any other kind of light. If this chunk of mass wasn't gas, then it must be dark matter.
Normally, the regular stuff – gas and stars – is embedded in huge halos of the dark stuff, all held together with gravity. But an enormous collision like that of the Bullet Cluster can strip the two apart.
When the two clusters collide, the clouds of gas crash and slow each other down. But because dark matter hardly interacts with anything, it keeps going, passing through the other cluster like a ghost, leaving the gas behind. Because astronomers happen to be catching the Bullet in mid-collision, their measurements have caught the moment that the dark matter has begun outrunning the gas. For the first time, astronomers detected dark matter in naked isolation.
As a concept, dark matter isn't new. Scientists have hypothesised its existence since the 1930s.
It was really clear that the properties of the matter we saw are unlike anything that we know of here on Earth
While most think it's real, almost every line of evidence has been indirect. For example, some galaxies are spinning so fast that they should be flying apart. Something like dark matter must be tugging on the stars, holding the galaxy together. Without direct evidence, however, there was some wiggle room for whether the stuff actually existed.
Some researchers suggest that you don't need a mysterious kind of matter to explain the curious observations. Instead, they say, maybe the laws of gravity are wrong, behaving differently at large scales. If you modify gravity, you can account for the weird phenomena that hint at dark matter.
"But when the Bullet Cluster results came along, it was really clear that the properties of the matter we saw are unlike anything that we know of here on Earth," Bradac says. Those modified theories of gravity couldn't explain why a big component of matter was separate from the gas. "Because it was so clearly separated, all those theories of modifying gravity were basically excluded."
Some still contend that an unknown kind of neutrino could make up the invisible mass in the Bullet Cluster. But that remains a minority opinion. "Most of the astronomical community believes this is some of the strongest evidence for dark matter," Kravtsov says.
But just knowing dark matter exists isn't enough. Scientists want to know what it is. They hypothesise that it must be made from some entirely unknown particle, or class of particles, that doesn't interact with normal particles or produce any kind of radiation – explaining why it is so hard for us to identify. In fact, according to the most popular theories, these dark matter particles don't even interact with each other.
Despite concerted efforts, however, researchers have yet to detect the existence of such particles, prompting some to propose new ideas. Maybe, they say, dark matter particles can self-interact after all, colliding and ricocheting off one another like billiard balls. In fact, there could even be a whole group of dark particles that interact just like regular matter. There could be dark protons, dark electrons, and even dark photons.
One way to determine whether dark matter interacts with itself – and if so, how strongly – is to slam the particles together. Do they zip right through each other, or do they bounce off?
We can actually put a number on the properties of dark matter
Fortunately for scientists, they don’t have to go to the trouble of building a dark matter particle collider, because those collisions are happening naturally in the Bullet Cluster. The galaxies in the two merging clusters are embedded in halos of dark matter, so when the clusters collide, so does the dark matter.
Observations so far suggest that, in the Bullet Cluster, the dark matter doesn't seem to interact with itself very much. That doesn't necessarily mean dark matter can't interact, but it places a limit on the strength of those interactions. Because the collision is clearly observed and well defined, the Bullet provides some of the best measurements yet, Bradac says.
"We can actually put a number on the properties of dark matter," she says. "That's something that hasn't really been done before."
But there's room for improvement. As researchers continue to refine their computer simulations of the Bullet, they can figure out in more detail what's going on and measure the properties of dark matter even better.
Of course, other data are needed. Bradac is part of a US-based team that's studying 25 other galaxy cluster collisions. The hope is that in a few years, after the analysis is complete, the researchers will know definitively whether or not dark matter is self-interacting.
It's one of the few cases where we see a clear cosmic shock
But while dark matter is the Bullet Cluster's claim to fame, that's not all it's about. Bradac has literally moved past the cluster, probing the galaxies behind it.
Because the cluster is so massive, its gravity bends light rays emanating from background galaxies – the same phenomenon that allowed astronomers to weigh the cluster. This gravitational lensing effect turns the Bullet Cluster into a giant magnifying glass that brings background galaxies into closer view.
Those distant background galaxies are important because they're among the first that ever formed. The Bullet Cluster thus offers a glimpse into the young Universe and how galaxies came to be. Bradac is part of a project to survey other clusters, using them as lenses to study the first galaxies.
As for the Bullet Cluster itself, it's helping physicists understand the basic behaviour of hot, electrically charged gas called plasma. It's a unique example of how two galaxy clusters can produce huge shockwaves. "It's one of the few cases where we see a clear cosmic shock," Kravtsov says.
These shocks exist in the solar system, too. The sun spews out charged particles called the solar wind. When the particles hit Earth's magnetosphere, a protective bubble formed by the planet's magnetic field, they generate a shock.
In the next several years, these instruments may find up to tens of thousands of clusters
Compared to a cluster, the solar system's plasma is denser and in the presence of strong magnetic fields. But despite these differences, studying the physics of plasma in the Bullet can provide insight about plasma in the solar system and elsewhere.
Clearly, galaxy clusters like the Bullet are incredibly useful. They are relatively rare, Kravtsov says, as clusters of such hulking size only experience major collisions once or twice in their entire lives.
But astronomers are scanning the skies with two telescopes in Chile – the Dark Energy Camera on the Blanco Telescope and the Atacama Cosmology Telescope – to hunt for more clusters. These surveys will find the biggest ones, above a hundred trillion times as massive as the sun, of which there could be hundreds of thousands in the Universe.
In the next several years, these instruments may find up to tens of thousands of clusters, including others like the Bullet Cluster. Until then, though, the Bullet remains one of a kind.