It’s in the room with you now. It’s more subtle than the surveillance state, more transparent than air, more pervasive than light. We may not be aware of the dark matter around us (at least without the ingestion of strong hallucinogens), but it’s there nevertheless.

Although we can't see dark matter, we know a bit about how much there is and where it's located. Measurement of the cosmic microwave background shows that 80 percent of the total mass of the Universe is made of dark matter, but this can’t tell us exactly where that matter is distributed. From theoretical considerations, we expect some regions—the cosmic voids—to have little or none of the stuff, while the central regions of galaxies have high density. As with so many things involving dark matter, though, it’s hard to pin down the details.

Unlike ordinary matter, we can’t see where dark matter is by using the light it emits or absorbs. Astronomers can only map dark matter's distribution using its gravitational effects. That’s especially complicated in the denser parts of galaxies, where the chaotic stew of gas, stars, and other forms of ordinary matter can mask or mimic the presence of dark matter. Even in the galactic suburbs or intergalactic space, dark matter’s transparency to all forms of light makes it hard to locate with precision.

Despite that difficulty, astronomers are making significant progress. While individual galaxies are messy, analyzing surveys of huge numbers of them can provide a gravitational map of the cosmos. Astronomers also hope to overcome the messiness of galaxies and estimate how much dark matter must be in the central regions using careful observation of the motion of stars and gas.

There's also been a tantalizing hint of dark matter particles themselves in the form of a signal that may come from their annihilation near the center of the Milky Way. If this is borne out by other observations, it could constrain dark matter's properties while avoiding messy gravitational considerations. Adding it all up, it's a promising time for mapping the location of dark matter, even as researchers still build particle detectors to identify what it is.

A (very) brief history of dark matter

In the 1930s, Fritz Zwicky measured the motion of galaxies within the Coma galaxy cluster. Based on simple gravitational calculations, he found that they shouldn’t move as they did unless the cluster contained a lot more mass than he could see. As it turned out, Zwicky’s estimates of how much matter there was were too large by a huge factor. Still, he was correct in the broader picture: more than 80 percent of a galaxy cluster’s mass isn’t in the form of atoms.

Zwicky’s work didn’t get a lot of attention at the time, but Vera Rubin’s later observations of spiral galaxies were another matter. She found that the combined stars and gas had too little mass to explain the rotation rates she measured. Between Rubin’s work and subsequent measurements, astronomers established that every spiral galaxy is engulfed by a roughly spherical halo (as it is called) of matter—matter that's transparent to every form of light.

The Bullet Cluster

That leads us to the “Bullet Cluster,” one of the most important systems in astronomy. First described in 2006, it’s actually a pair of galaxy clusters observed in the act of colliding. Researchers mapped it in visible and X-ray light, finding that it consists of two clumps of galaxies. But it's the stuff they couldn't image directly that ensured the Bullet Cluster is rightfully cited as one of the best pieces of evidence for dark matter’s existence (the title of the paper announcing the discovery even calls it “direct empirical proof”).

Galaxy clusters are the biggest individual objects in the Universe. They can contain thousands of galaxies bound to each other by mutual gravity. However, the stuff within those galaxies—stars, gas, dust—is outweighed by an extremely hot, gaseous plasma between them, which shines brightly in X-rays. In the Bullet Cluster, the collision between the two clusters created a shock wave in the plasma (the shape of this shock wave gives the structure its name).

More dramatically, though, the astronomers who described the cluster used gravitational lensing—the distortion of light from more distant galaxies by the mass within the cluster—to map the distribution of most of the material in the Bullet Cluster. That method is known as "weak gravitational lensing." Unlike the sexier strong lensing, weak lensing doesn’t create multiple images of the more distant galaxies. Instead, it slightly warps the light from background objects in a small but measurable way, depending on the amount and concentration of mass in the “lens”—in this case, the cluster.

Astronomers found the shocked plasma, which represents most of the mass of the Bullet Cluster, was almost entirely in the region between the two clusters, separated from the galaxies. However, the mass was largely concentrated around the galaxies themselves. This enabled a clear, independent measurement of the amount of dark matter, separate from the mass of the gas.

The results also confirmed some predictions about the behavior of dark matter. Thanks to the shock of the collision, the plasma stayed in the region between the two clusters. Since the dark matter doesn’t interact much with either itself or normal matter, it passed right through the collision without any noticeable change.

It’s a phenomenal discovery, but it’s only one galaxy cluster, and that ain’t enough. Science is inherently greedy for evidence (as it should be). A single example of anything tells us very little in a Universe full of possibilities. We want to know if dark matter always clusters around galaxies or if it can be more widely dispersed. We want to know where all the dark matter is, in all galaxy clusters and beyond, throughout the entire cosmos.