What Is Dark Matter, and How Do Scientists Search for It?

Look up at a clear night sky and almost everything visible, stars, planets, glowing nebulae, is made of ordinary matter: protons, neutrons and electrons. But according to decades of astronomical evidence, that familiar matter is a minority. Roughly five times more of the universe's mass appears to be something else entirely, a substance that neither emits nor absorbs light and has never been directly detected. Astronomers call it dark matter.

How Astronomers Know It Is There

Nobody has seen dark matter directly, but its gravity leaves fingerprints everywhere. In the 1970s, astronomer Vera Rubin found that stars at the edges of spiral galaxies orbit almost as fast as stars near the center, something that should be impossible if a galaxy's mass matched its visible stars and gas. The outer stars should fly off into space unless a huge amount of unseen mass, spread through and around the galaxy, is holding them in orbit with extra gravity.

Similar evidence keeps showing up elsewhere. Clusters of galaxies bend light from more distant objects behind them, an effect called gravitational lensing, and they bend far more light than their visible matter can explain. Maps of the cosmic microwave background, the faint afterglow of the Big Bang, show patterns that only fit if roughly 27% of the universe's total mass and energy is dark matter, compared with about 5% ordinary matter. The rest is dark energy, a separate and even less understood phenomenon.

What Dark Matter Probably Is Not, and Might Be

Dark matter is not simply matter that is hard to see, like dim stars, dust clouds or black holes. Astronomers have largely ruled that out because there is not nearly enough ordinary matter, dim or otherwise, to account for the gravity observed. Instead, most physicists suspect dark matter is made of a new type of particle that barely interacts with ordinary matter or light at all, only through gravity and possibly one other very weak force.

Leading candidates include WIMPs, or weakly interacting massive particles, and axions, an extremely light hypothetical particle. Neither has been confirmed. Some more exotic proposals suggest dark matter might not be a particle at all, but a modification to how gravity itself behaves at galactic scales, though this idea explains the evidence less completely than particle-based models.

How Scientists Try to Find It

Three main strategies are underway. Direct-detection experiments, often built deep underground to shield them from cosmic rays, wait for a dark matter particle to collide with an ordinary atom and produce a tiny, measurable signal. Indirect-detection experiments look to space for the aftermath of dark matter particles colliding with each other, which some theories predict would release detectable radiation. Collider experiments, such as those at the Large Hadron Collider, try to create dark matter particles briefly by smashing ordinary particles together at high energy.

A newer approach uses astronomical structures as gravity probes. The way stars, gas or entire dwarf galaxies move under gravity's pull can reveal how much unseen mass surrounds them, without needing to catch a dark matter particle at all. This method has traditionally worked best inside or near the Milky Way, where astronomers can study the relevant structures in detail.

Why the Mystery Persists

Decades of experiments have narrowed down what dark matter is not, without pinning down what it is. That combination, overwhelming evidence for its existence alongside a complete absence of direct detection, makes dark matter one of the most stubborn open problems in modern physics, and one that touches nearly every galaxy in the observable universe.