Everything we can see, every star, planet and glowing cloud of gas, accounts for only about 15 percent of the matter in the universe. The rest is dark matter, an invisible substance that reveals itself only through gravity, holding galaxies together and sculpting the cosmic web. It does not shine, absorb or reflect light; it passes through ordinary matter and through itself like a ghost. After nearly a century of evidence, scientists are confident it exists, yet no one has directly detected a single particle of it.

The evidence that won’t go away

The case for dark matter rests on several independent pillars. In the 1930s Fritz Zwicky noticed that galaxies in clusters move too fast to be held by visible mass alone. In the 1970s Vera Rubin measured the rotation of spiral galaxies and found that their outer stars orbit far too quickly; by all rights they should fly off into space, unless embedded in a vast halo of unseen mass. Gravitational lensing, where massive objects bend light from behind them, reveals far more bending than visible matter can explain. And the cosmic microwave background, the afterglow of the Big Bang, carries an imprint that only makes sense if dark matter outweighs normal matter five to one.

What dark matter does

Dark matter is the universe’s scaffolding. In the early universe, its gravity pulled ordinary matter into filaments and knots, and galaxies formed where the dark matter was densest. Without it, the tiny ripples seen in the Big Bang’s afterglow could never have grown into the galaxies we see today; ordinary matter alone, pushed around by radiation pressure, could not have clumped fast enough. Computer simulations of cosmic structure only reproduce the observed universe when dark matter is included. In a very real sense, we owe our existence to stuff we cannot see.

What could it be?

The leading suspect has long been the WIMP, the weakly interacting massive particle, a hypothetical particle that would feel gravity and the weak nuclear force but barely interact otherwise. Decades of ever-more-sensitive underground detectors have hunted WIMPs and found nothing, progressively ruling out the most favoured models. These experiments sit deep underground, shielded from cosmic rays, watching tonnes of ultra-pure liquid xenon for a single telltale flash of a dark matter collision. Attention has broadened to other candidates:

• Axions, ultra-light particles originally proposed to solve a puzzle in particle physics.
• Sterile neutrinos, heavier cousins of the familiar neutrino that barely interact at all.
• Primordial black holes, hypothetical black holes formed in the Big Bang’s first moments.

Could the theory be wrong instead?

A minority of physicists argue that dark matter is a sign our theory of gravity is incomplete. Modified Newtonian Dynamics, or MOND, proposes that gravity behaves differently at the tiny accelerations found in galaxies’ outskirts, which could explain rotation curves without any invisible matter. MOND scores some impressive successes with individual galaxies but struggles with galaxy clusters and the cosmic microwave background, where dark matter fits beautifully. Most cosmologists consider dark matter the far stronger hypothesis, but the debate keeps both sides honest.

FAQs

How do we know dark matter is real? Multiple independent lines of evidence, galaxy rotation, gravitational lensing and the cosmic microwave background, all point to the same conclusion: unseen mass outweighs visible matter about five to one.

Is dark matter the same as dark energy? No. Dark matter clumps under gravity and holds structures together; dark energy is a mysterious repulsive effect driving the universe’s accelerating expansion. They are two separate unknowns.

Could dark matter be passing through me now? If it is made of WIMPs, billions of particles may stream through your body every second without a single interaction. That is exactly why it is so hard to detect.

Will we ever find it? New detectors, the Large Hadron Collider and astronomical surveys are all hunting. A discovery would be the biggest physics breakthrough in decades.

Dark matter is a reminder of how much remains unknown: we have mapped the invisible scaffolding of the cosmos in exquisite detail without knowing what it is made of. The substance that built every galaxy is still waiting for its identity to be revealed. When the answer finally comes, whether from a deep mine, a particle collider or a telescope, it will likely rewrite physics as profoundly as the discovery of the atom did.

Source: CERN