Not long after physicists on experiments at the Large Hadron Collider at CERN laboratory discovered the Higgs boson, CERN Director-General Rolf Heuer was asked, “What’s next?” One of the top priorities he named: figuring out dark matter.
Dark matter is five times more prevalent than ordinary matter. It seems to exist in clumps around the universe, forming a kind of scaffolding on which visible matter coalesces into galaxies. The nature of dark matter is unknown, but physicists have suggested that it, like visible matter, is made up of particles.
Dark matter shows up periodically in the media, often when an experiment has spotted a potential sign of it. But we are still waiting for that Nobel-Prize-triggering moment when scientists know they finally have it.
Here are four facts to get you up to speed on one of the most exciting topics in particle physics:
1. We have already discovered dark matter
Illustration by Sandbox Studio, Chicago
At this moment, several experiments are on the hunt for dark matter. But scientists actually discovered its existence decades ago.
In the 1930s, astrophysicist Fritz Zwicky was observing the rotations of the galaxies that form the Coma cluster, a group of more than 1000 galaxies located more than 300 million light years from Earth. He estimated the mass of these galaxies, based on the light they emitted. He was surprised to find that, if this estimate were correct, at the speed at which the galaxies were moving, they should have flown apart. In fact, the cluster needed at least 400 times the mass he had calculated to hold itself together. Something mysterious seemed to have its finger on the scale; an unseen “dark” matter seemed to be adding to the mass of the galaxies.
The idea of dark matter was largely ignored until the 1970s, when astronomer Vera Rubin saw something that gave her the same thought. She was studying the velocity of stars moving around the center of the neighboring Andromeda galaxy. She anticipated that the stars at the edge of the galaxy would move more slowly than those at its axis because the stars closest to the bright—and therefore massive—cluster of stars in the center would feel the most gravitational pull. However, she found that stars on the margins of the galaxy moved just as quickly as those in the middle. This would make sense, she thought, if the disc of visible stars were surrounded by an even larger halo made of something she couldn’t see: something like dark matter.
Other astronomical observations have since confirmed that something strange is going on with the way galaxies and light move through space. It’s possible that our confusion stems from a flaw in our understanding of gravity—Rubin herself said she favors this idea. However, if it’s true that dark matter exists, we’ve already seen its effects.
2. We have possibly already observed dark matter
Several experiments are searching for dark matter, and some of them may have even already found it. The problem is that no experiment has been able to make that claim with enough confidence to convince the wider scientific community—either due to statistics or an inability to rule out alternative possible explanations. And no two claims have lined up quite convincingly enough for scientists to declare any result confirmed.
In 1998 scientists on the DAMA experiment, a dark matter detector buried in Italy’s Gran Sasso mountain, saw a promising pattern in their data. The rate at which the experiment detected hits from possible dark matter particles changed over the course of the year—climbing to its peak in June and dipping to its nadir in December.
This was exactly what DAMA scientists were looking for. If our galaxy is surrounded by a dark matter halo, the Earth is constantly moving through that halo as it orbits the sun—and the sun is constantly moving through the dark matter as it orbits the center of the Milky Way. During half of the year, the Earth is moving in the same direction as the sun. During the other half, it is moving in the opposite direction. When the Earth and the sun are moving in tandem, their combined velocity through the dark matter halo is faster than the Earth’s velocity when it and the sun are at odds. DAMA’s results seemed to reveal that the Earth really was moving through a dark matter halo.