Dark Matter Unveiled: From Galaxy Rotation to the Cosmic Microwave Background

Short summary

In this Royal Institution talk, a leading astrophysicist explains why the majority of the universe is thought to be dark matter, an invisible substance detected only through gravity. Beginning with galaxy rotation curves and Vera Rubin's iconic measurements, the talk then covers strong and weak gravitational lensing to map unseen mass, discusses historical hints from Fritz Zwicky, and highlights the bullet cluster as a decisive test. It also surveys the cosmic microwave background as a relic imprint that constrains dark matter's abundance, and surveys candidate particles such as WIMPs and axions along with ongoing detection efforts. The lecture emphasizes the scientific journey and the open question of what dark matter actually is.

Overview and Opening Thought

The talk opens with a provocative premise: that most of the universe is made of something we cannot directly see. The presenter frames the investigation as a methodical pursuit of evidence, beginning with galaxies and using gravity as a diagnostic tool to infer the mass that must be present even when it is not luminous. The aim is to convince the audience that the conventional picture of visible matter alone cannot account for the observed motions and structures in the cosmos.

Weighing the Unseen: Gravity as a Tool

The speaker emphasizes gravity as the essential instrument to weigh astronomical objects. By analyzing orbital dynamics, such as a planet or star around a center of mass, one can deduce the mass responsible for the observed gravitational influence. The Earth around the Sun, the orbit of stars near the Galactic center, and the motion of stars in distant galaxies share this logic. The key rules are intuitive: mass increases orbital speed, while increasing distance from the mass decreases speed. This framework allows astronomers to quantify the total mass of a system without needing to see all of its constituents.

Galaxy Rotation Curves and Vera Rubin

Moving to galaxies, the talk explains that galactic rotation curves show stars at large radii orbiting at similar speeds to inner stars, contrary to expectations if only visible matter were present. Vera Rubin’s insight proposed a conceptual image of galaxies embedded in invisible halos of dark matter, explaining the flat rotation curves as evidence for unseen mass. This led to a grand shift in astrophysics, expanding the problem from clusters to every galaxy, including our Milky Way and the Sun’s neighborhood.

The talk contrasts two competing explanations: dark matter as an unseen halo of material, and MOND as a modification of gravity itself. MOND sought to reproduce galaxy dynamics by tweaking gravity at very low accelerations, but the wider set of observations soon favored dark matter as the more robust explanation across cosmic scales.

From Clusters to Gravity: Fritz Zwicky and Early Hints

The narrative recounts Fritz Zwicky’s observations of the Coma cluster in the 1930s, where galaxies moved too fast to be bound by visible matter alone. He speculated about unseen mass, a notion that foreshadowed later discoveries though the solution remained unresolved for decades. X-ray observations of clusters showed hot gas that added mass, yet still left a discrepancy that dark matter would eventually address.

Gravitational Lensing: Mapping Mass with Light

The discussion transitions to gravitational lensing as a direct probe of mass distribution. Strong lensing produces spectacular distortions and Einstein rings, while weak lensing reveals subtle statistical shearing of background galaxies. Both approaches enable mass maps that frequently align with galaxies but also reveal mass where little light exists, consistent with dark matter halos.

The Bullet Cluster: A Turning Point

One of the most compelling tests is the bullet cluster, where two clusters collide. In this event, the galaxies behave like collisionless particles, while the gas experiences friction and slows. Weak lensing shows the bulk of mass offset from the gas, co-locating with the galaxies, precisely what dark matter predicts and challenging modified gravity scenarios that lack an independent dark matter component.

Independence of Evidence: Gravitational Lensing and the CMB

Beyond local dynamics, the talk highlights independent lines of evidence, including gravitational lensing maps across the universe and the cosmic microwave background (CMB). The CMB power spectrum, measured with Planck data, encodes primordial fluctuations that, when modeled, yield a cosmological composition that includes a substantial dark matter fraction. The speaker demonstrates how modeling all scales, from large to small, yields a consistent cosmology with roughly 5% normal matter, 27.5% dark matter, and 67.5% dark energy, a ratio that elegantly matches structure formation observations and lensing results. Neutrinos create a background limit that constrains detection efforts, illustrating how multiple probes converge on a coherent picture of the cosmos.

Candidates and Detection Strategies

The talk surveys what dark matter could be made of, outlining the historical WIMP hypothesis alongside MACHOs, both of which have faced stringent constraints. WIMPs offer a particle that interacts weakly with normal matter and gravity, but direct detection experiments like Xenon-based detectors stacked deep underground have yet to observe a definitive signal. The concept of the neutrino fog shows the ultimate sensitivity limit, as solar neutrinos begin to mimic the rare dark matter interactions that detectors seek. Given these challenges, axions emerge as an appealing alternative, with tabletop experiments capable of probing their existence through resonant mechanisms that avoid requiring massive underground facilities. The breadth of candidates highlights the scientific openness and ongoing thrill of discovery in this field.

Current Status and Future Prospects

The speaker emphasizes that while the evidence for dark matter is compelling across multiple, independent lines of inquiry, a definitive particle has not yet been detected. The LHC recovered the Higgs boson, but the hunt for dark matter continues with a broader spectrum of experimental approaches. The talk conveys a sense of anticipation about what the future holds as more sensitive detectors, novel techniques, and novel candidates push the boundary of knowledge. This is science in action: a dynamic, convergent process that uses gravity, light, and the early universe to illuminate the unseen mass shaping cosmic history.