What Is Dark Matter Day? Scientists Launch First-Ever Event Celebrating Universe's Biggest Mystery

milky way
The Milky Way galaxy. ESA/Hubble & NASA

As the rest of the world marks Halloween, scientists from across the globe will be celebrating something far more eerie—dark matter.

This October 31 marks the inaugural Dark Matter Day, an event launched by Interactions Collaboration, a group of particle physics communication specialists hoping to raise awareness about the elusive substance and the experiments currently taking place to find and understand it.

Scientific institutions around the world, including CERN, the European Organization for Nuclear Research, London's Royal Astronomical Society, as well as institutions across the U.S., are holding events in honor of the day, the details of which can be found here.

What is dark matter?

Normal matter—the stuff that makes up everything we can see in the universe, like stars and planets, makes up just 5 percent of the universe. Most of the universe—about 68 percent—is dark energy, the unseen force thought to be driving the expansion of the universe. The rest, around 27 percent, is dark matter.

Our knowledge of dark matter dates back to the 19th century, when astronomers started noticing inconsistences in what they were seeing versus what should be there. Dutch astronomer Joacobus Kapteyn first suggested the existence of "dark matter" in the 1920s, and in the decades that followed more scientists began to give weight to the idea of the universe's unseen matter.

We now know that dark matter exists because of the gravitational influence it has on galaxies. Galaxies, including our own Milky Way, rotate. Dark matter slows this rotation—if it wasn't there, they would spin themselves into oblivion.

However, attempts to observe dark matter have proven futile. It is, by nature, dark, meaning we cannot see it—it does not absorb, reflect or emit light. Understanding what dark matter is made of would help scientists answer fundamental questions about the universe.

What are scientists doing to find dark matter?

Experiments are currently underway in locations across the globe to try to trace dark matter. These normally involve detectors buried deep underground, where interactions from outside influences are limited.

This includes the IceCube Neutrino Observatory in Antarctica, where scientists look for dark matter by detecting neutrinos—cosmic particles that only interact with weak subatomic force and gravity.

One of the main hypotheses in the hunt for dark matter is that it is made of weakly interacting massive particles (WIMPs). These hypothetical particles interact via gravity and forces that are not currently part of the standard model of particle physics. Scientists think that with a high enough density, WIMPs would annihilate each other and decay into neutrinos. Evidence of this is yet to be found, however.

What would discovering dark matter mean?

black hole
Artist impression of a black hole. NASA/SOFIA/Lynette Cook

Understanding the nature of dark matter would help scientists fill out the standard model—it would provide a major piece of the jigsaw puzzle that is the universe we live in. By getting a handle on dark matter, we would be able to start answering other unanswered questions, like what happened just after the Big Bang, and how the universe came to be as it is today. One controversial theory has even linked dark matter to black holes.

As the organizers at Interactions Collaboration explain: "Dark matter is the glue that holds galaxies together, but we don't know what it is. Understanding its true nature could explain its origins, evolution, and overall structure in the universe."

What Is Dark Matter Day? Scientists Launch First-Ever Event Celebrating Universe's Biggest Mystery | Tech & Science