Nobody realized that something was missing until 1932.

That was the year that Jan Hendrik Oort made a startling discovery. The Dutch astronomer was measuring the speed of stars in our galaxy, and something was wrong with his calculations. Just as it keeps humans tethered to the Earth, and the Earth looping around the Sun, gravity keeps all the stars in a galaxy clustered together. Oort knew that any stars he observed in the galaxy were held in by gravity. But gravity depends on mass, and a lot of gravitational pull requires a lot of material. Yet when Oort measured the speed of the stars and added up all the mass he could see in our galaxy, he found there was too little mass to keep the stars from hurtling right out of the galaxy. In fact, he could only see only one-third of the mass that was necessary. Oort thought he must have failed to account for some dim or hidden stars.  

But a year later, Caltech astronomer Franz Zwicky ran into the same problem while observing the speed of galaxies in a galaxy cluster (a group of many galaxies, bound by gravity). This time, Zwicky found a mere one-tenth of the visible matter required to hold the cluster together. Zwicky decided there had to be mass present that couldn’t be seen—something taking up space, pulling galaxies together, but completely invisible. He called this material “dark matter.”

Other scientists were skeptical at first. But the evidence kept coming in. In the 1970s, Vera Rubin and Kent Ford observed stars in spiral galaxies, which appear to store most of their visible matter in their bright centers. Yet Rubin and Ford noticed that stars at the edges of the galaxy were traveling at the same speed as those closer to the luminous center. This didn’t make sense, because as two objects move away from each other, the pull of gravity between them quickly decreases. Bodies farther from a source of gravity should feel less pull and move more slowly: for example, Pluto orbits our Sun more slowly than Venus does. Rubin and Ford realized that another source of gravity was speeding the outer stars along—something spread out through the galaxy, rather than concentrated with the visible mass. This was dark matter.

Think of everything in existence that we can see: people, oranges, umbrellas, grains of sand. Consider beyond Earth to stars, planets, and the dim smudges of distant galaxies. These all fall into a class known as “normal matter.” Normal matter interacts with light energy, either generating its own light, or sucking up or bouncing back incoming light. But familiar particles like this are actually a rarity. Scientists predict that there is over five times as much dark matter in our universe as the normal matter we know and understand. Dark matter appears to interact only with the force of gravity.

So far, scientists can only measure this dark matter by calculating its effect on celestial entities. These include stars and galaxies, as Rubin, Zwicky, and others have shown, but it also includes light from distant stars, which gets bent off course by the gravity of galaxies it passes on its way toward Earth. Measurements of this “lensing” effect show that the gravitational force exerted by these galaxies is, again, far too great to be caused by their visible matter alone.

But what is dark matter? Scientists aren’t sure, but they do have many suggestions. One popular idea is that dark matter is made up of undiscovered theoretical particles. These might be lightweight particles called “axions,” or particles with high mass that don’t interact strongly with their neighbors, which are collectively known as “Weakly Interacting Massive Particles” or WIMPs. Complex physical theories accompany these potential dark matter candidates. For example, WIMPs could exist in a universe that has a fourth spatial dimension, or in a universe where every known particle has a theoretical partner with different properties.

Nobody has definitively found a WIMP yet, but dark matter detectors in Japan, Canada, Italy, South Dakota, and other locations aim to catch one as it passes through Earth. The goal is to record a rare event: a chance collision between a WIMP and the dense center, or nucleus, of an atom of normal matter. If this collision happens, the nucleus should “bounce,” and scientists hope to see the tiny amount of energy released by this motion as a pulse of sound or electricity. Some WIMP detectors are filled with sensitive superconducting metals, cooled to a temperature so low that this tiny particle collision would cause a measurable release of heat. To make sure that other particle interactions aren’t recorded instead, many of these WIMP detectors are buried far underground, away from any incoming radiation.

But some scientists think that WIMPs, and dark matter at large, may not exist at all. Instead, they think we might be using the wrong equations to calculate force in our universe. Making some small adjustments to Isaac Newton’s law of gravity, these researchers say, might be the correct way to adjust for differences between visible mass and measurable gravity in space.

So far, there’s no clear solution to the problem that first astonished Oort over seven decades ago. Whether theoretical particles or adjusted equations provide a final answer, the true nature of dark matter remains one of the biggest mysteries in the universe.