Dark Matter: The Invisible Glue Shaping the Cosmos

Dark matter is the unseen scaffolding of the universe, comprising about 27% of its mass and influencing the formation and behavior of galaxies through gravity. This overview explains what dark matter is, how scientists inferred its existence beginning with Fritz Zwicky in the 1930s and Vera Rubin in the 1970s, and how gravitational lensing and galaxy cluster collisions like the Bullet Cluster provide direct evidence. It also covers why scientists favor cold dark matter, outlines leading candidates such as WIMPs, axions, and primordial black holes, and clarifies how dark matter differs from dark energy. The piece closes with how future missions like the Nancy Grace Roman Space Telescope may map dark matter across the cosmos. Author: Chelsea Gohd, NASA's Jet Propulsion Laboratory.

What is Dark Matter?

Dark matter is the invisible substance that makes up most of the mass in galaxies and galaxy clusters. While normal matter accounts for roughly 5% of the universe, dark matter contributes about 27%, with the remainder being dark energy. Unlike ordinary matter, dark matter does not interact with light or the electromagnetic spectrum, yet its gravity influences the motion of stars and the structure of the cosmos. In essence, it acts as the invisible glue that holds the universe together and shapes cosmic evolution on the largest scales.

"Dark matter acts as invisible glue holding galaxies together" - Fritz Zwicky, astronomer

Discovering Dark Matter

The concept emerged in the 1930s when Fritz Zwicky observed the Coma Cluster and found galaxies moving too fast to be bound by the visible matter. He introduced the term dunkel Materie, now dark matter. In the 1970s Vera Rubin studied spiral galaxies and found that outer stars moved too quickly for the visible mass to hold them in place, implying a substantial amount of unseen matter. These early efforts established the case for dark matter as a real, pervasive component of the universe.

Studying Dark Matter

Modern evidence comes from gravity’s effects on light and matter. Gravitational lensing—where dark matter bends light from distant galaxies—allows scientists to map its distribution. A striking example is the Bullet Cluster 1E 0657-56, formed by a collision of two galaxy clusters. X-ray observations show hot gas (normal matter) in pink, while gravitational lensing reveals blue regions that trace the majority of mass, consistent with dark matter. This and similar observations provide direct evidence of dark matter’s existence and its separation from ordinary matter during cluster mergers.

Future maps of dark matter, aided by missions like NASA’s Nancy Grace Roman Space Telescope, will illuminate its distribution across the universe and help researchers understand its connection to cosmic expansion and dark energy.

Taking Dark Matter’s Temperature

Scientists consider dark matter to be cold or slow-moving, based on simulations that compare the formation of cosmic structures under cold versus warm or hot dark matter. The observed large-scale structure aligns best with cold dark matter, which would have formed early in the universe and allowed galaxies and clusters to accrue their present distribution.

Leading Dark Matter Candidates

Researchers propose several possible components, with WIMPs (Weakly Interacting Massive Particles), axions, and primordial black holes among the leading contenders. WIMPs are heavy particles that interact primarily through gravity, possibly generating gamma rays when they annihilate. Axions are light, low-energy particles theorized for fundamental physics reasons and searched for with X-ray and gamma-ray observations. Primordial black holes—formed shortly after the Big Bang—could also contribute to dark matter. It is possible that more than one type of matter or particle contributes to the dark sector, and ongoing experiments continue to test these candidates.

"WIMPs are a favored class of dark matter candidates" - Astronomy community

Dark Matter Versus Dark Energy

Dark matter and dark energy are distinct mysteries: dark matter clusters around galaxies and clusters, while dark energy drives the accelerated expansion of the universe. Mapping dark matter helps researchers understand the structure of the universe and its evolution in relation to dark energy, advancing our overall cosmological picture. The ongoing research aims to connect the mass distribution from dark matter with the cosmic expansion influenced by dark energy.

Author: Chelsea Gohd, NASA's Jet Propulsion Laboratory