What is dark matter? Explore the possibilities in the Big Ideas Lab podcast

Galaxies spin faster than they should. Clusters of those galaxies hold themselves intact against all expectations. By every visible measure, the universe should not look the way it does.

Something is holding galaxies, stars and entire clusters together. Something we can’t see. And although it’s invisible to the naked eye, there are clues everywhere that allude to its presence. Learn more about that mysterious missing mass — dark matter — on the latest episode of the Big Ideas Lab. Listen on Apple or Spotify.

Dark matter is a hidden, invisible substance threaded throughout the cosmos. It makes up a staggering portion of the total mass present in the universe — about 85%. Yet it remains hidden from view, difficult to observe and even harder to understand.

“The only way that we can really infer that it's there is because it interacts with the stuff that we can see,” said Lawrence Livermore National Laboratory (LLNL) scientist Greg Sallaberry.

For nearly a century, scientists have explored a wide range of possibilities for what dark matter might be. One early idea focused on Massively Compact Halo Objects (MACHOs), enormous, heavy objects like black holes or brown dwarfs hiding in the outskirts of galaxies.

Dark matter could also be made of tiny, elusive particles drifting through space. But their unknown mass makes it nearly impossible to know where or how to look for them.

It’s like charting an invisible ocean current: you can see the tug on galaxies, watch the cosmos bend, but the source of the flow remains invisible.

In the early 1990s, scientists at LLNL joined the search for dark matter through the MACHO survey. As the data rolled in, a startling truth emerged: MACHOs were out there, silently drifting through the Milky Way, but not nearly enough to explain the universe’s hidden mass.

Now, the new Vera C. Rubin Observatory’s Legacy Survey of Space and Time continues to search for dark matter at the largest scales. But Lawrence Livermore scientists are also looking for answers at the opposite extreme — minuscule particles that could be hiding as one of the universe’s most elusive ghosts: axions.

An axion is a hypothetical subatomic particle — and one of the leading candidates for what dark matter might be. They're believed to interact very weakly with ordinary matter, and yet also be abundant and pervasive, passing through all of space, even Earth itself, almost without a trace.

The Axion Dark Matter Experiment, or ADMX, uses a powerful magnetic field to nudge these invisible particles into revealing themselves as faint microwave signals.

“Our mantra has always been we want to either discover, or if not discover, rule out the axion over a certain mass range. We want to be able to scan that,” said LLNL scientist Gianpaolo Carosi.

Together, these approaches are pushing the boundaries of discovery, inching us closer to uncovering the hidden mass shaping everything we see. They speak to something deeply human: the desire to understand what lies beyond our reach.

Explore one of the most fundamental questions in science, “What is dark matter?” on the latest episode of the Big Ideas Lab on Apple or Spotify.