The universe contains a profound mystery: dark matter. This substance remains unseen, yet exerts a powerful influence over the cosmos. Scientists estimate this invisible material makes up the vast majority of matter in the universe, acting as the scaffolding upon which galaxies and larger structures are built. The question of whether we can physically interact with it depends on understanding the forces that govern all physical interaction.
What Dark Matter Is and Is Not
Dark matter is defined by its lack of interaction with light, which is why it earned the name “dark.” Cosmological models estimate that dark matter accounts for about 85% of all matter in the universe, vastly outweighing the ordinary, visible matter that forms stars, planets, and people. This substance is believed to be composed of a new type of particle not included in the Standard Model of particle physics.
Dark matter is not the same as dark energy, which is a separate phenomenon driving the accelerated expansion of the universe. It is also not simply unlit baryonic matter, like black holes or dark gas clouds, because the total amount of normal matter is constrained by observations of the early universe. This material represents an entirely new form of mass that interacts with its surroundings in a highly restricted manner.
The Role of the Electromagnetic Force in “Touching”
The reason any human can “touch” an object, or even see it, is entirely dependent on the electromagnetic (EM) force. When a hand presses against a surface, electrons in the atoms of the hand repel the electrons in the atoms of the surface, creating the sensation of solidity and resistance. Seeing an object relies on photons—packets of the EM force—bouncing off or being emitted by its atoms and traveling to the eye.
Dark matter particles do not carry an electric charge, which is necessary for electromagnetic interactions. Lacking this charge, dark matter can neither absorb, reflect, nor emit light, making it transparent to all telescopes. It cannot form the electron-shell structure of atoms, nor can it engage in the electron-to-electron repulsion that creates the sensation of touch. Consequently, dark matter passes through ordinary matter, including the human body and the Earth, as if it were not there.
Gravitational Clues: Indirect Evidence of Dark Matter
Although dark matter is electromagnetically silent and physically intangible, its existence is inferred through the one force it reliably exerts: gravity. The most compelling evidence comes from observations of galaxy rotation curves. Stars and gas clouds at the outer edges of spiral galaxies orbit the galactic center at nearly the same speed as objects closer in.
If the galaxy’s mass consisted only of visible stars and gas, the orbital velocity of the outer stars should decline sharply with distance, following Newtonian physics. The observed “flat” rotation curve indicates a massive, invisible halo of dark matter surrounds the galaxy, providing the gravitational pull needed to keep the faster-moving outer material from flying apart. Gravitational lensing provides another line of evidence, where the immense mass of galaxy clusters bends and distorts the light traveling from distant background galaxies. This effect confirms that the total mass of the cluster is far greater than the mass of the visible gas and stars.
The Active Search for Direct Interaction
Scientists have launched an active search to detect dark matter through a different, subtle force, rather than relying solely on indirect gravitational evidence. The hypothesis is that dark matter might interact with ordinary matter via the weak nuclear force, or perhaps an entirely new “dark force.” This interaction would be extremely rare and fleeting, but detectable with sensitive equipment.
The search focuses on two leading candidates: Weakly Interacting Massive Particles (WIMPs) and axions. WIMP detectors, often buried deep underground, are shielded from cosmic radiation and contain materials like liquid xenon. The goal is to catch the tiny recoil energy released if a WIMP particle collides with an atomic nucleus in the detector. Other experiments use strong magnetic fields to search for the predicted conversion of hypothetical axion particles into detectable photons.