Dark Matter
Dark matter is the invisible glue that holds the universe together. This mysterious material is all around us, making up most of the matter in the universe.
Dark matter makes up most of the mass in galaxies and galaxy clusters. In fact, scientists estimate that ordinary matter makes up only about 5% of the universe, while dark matter makes up about 27%. (The rest is thought to be dark energy, which is its own mystery). It's thought that dark matter shapes the cosmos, organizing galaxies and cosmic objects on a large scale.
From stars and galaxies to the shoes on your feet, ordinary matter makes up everything we can see in the universe — in wavelengths spanning from the infrared to visible light and gamma rays. While dark matter interacts with ordinary matter through gravity, it does not seem to interact at all with the electromagnetic spectrum, including visible light. So dark matter doesn't absorb, reflect, or emit any light.
While dark matter is invisible, it does have some things in common with ordinary matter: It takes up space and it holds mass. Because of this, we can see how it interacts with and influences ordinary matter throughout the universe, which is how we're able to "see" and study dark matter.
History of Discovery
While the term dark matter was mentioned in earlier publications, the current concept of dark matter materialized in the early 1930s. In 1933, Swiss-born astronomer Fritz Zwicky published a paper in which he suggested that there might be an invisible form of matter that created the gravity holding these galaxies together. He dubbed this mysterious material "dunkle Materie," which is German for dark matter.
While these early investigations sparked ideas and curiosity around dark matter, it was still seen as a fringe concept without sufficient evidence to support it.
That changed in the 1970s when American astronomer Vera Rubin observed this "missing matter" problem in spiral galaxies concluded that these galaxies must be held together by dark matter.
Rubin's discovery provided such strong evidence for dark matter that the concept was embraced by the scientific community. Today, while not all astronomers agree on what dark matter might be, its existence is widely accepted.
Observation
Today, scientists have even more direct evidence of dark matter. While dark matter doesn't interact with light, its gravity can bend light from distant galaxies, creating an effect called gravitational lensing. Studying galaxies distorted by gravitational lensing can help scientists better understand dark matter and its place in the universe.
In 2006, scientists observed the Bullet Cluster and discovered some of the best direct evidence for dark matter. This galaxy cluster, formally known as 1E 0657-56, was created when two large galaxy clusters collided in an extremely energetic event about 3.8 billion light-years from Earth.
During this collision, hot gas from one cluster interacted with hot gas from the other. In the image below, hot X-ray emitting gas made of normal matter and detected by NASA's Chandra X-ray Observatory is shown in pink. The blue portions show the distribution of dark matter and were revealed using gravitational lensing observations taken by NASA’s Hubble Space Telescope and the Giant Magellan Telescope, which is operated by an international consortium. The blue areas represent most of the mass in these clusters and are distributed differently than the hot gas. Researchers think that this material is likely dark matter. So, in this image, you can see direct evidence of dark matter with your own eyes.
Temperature
Because dark matter has mass, it also must have a temperature.
For a while, scientists considered two options — cold dark matter made up of slow-moving particles and warm or hot dark matter made of faster-moving particles.
Based on such simulations, scientists currently agree that it is most likely that whatever dark matter is, it is cold or slow-moving. It's additionally thought that this cold dark matter formed in the early universe with a low enough velocity to allow galaxies to form and distribute as we see them today.
Leading Dark Matter Candidates
WIMPs are hypothetical particles that are big, heavy, and slow-moving. They don't absorb or emit light or strongly interact with any other particles that we've seen so far.
Axions are hypothetical subatomic particles scientists think are both low-mass and low-energy. This particle was first theorized in 1977 as a solution to a fundamental problem within particle physics called the strong CP problem.
Black holes are the densest objects in the universe with gravity so extreme that at a certain point not even light can escape. Primordial black holes are hypothetical black holes scientists think formed right after the birth of the universe.
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