Dark Matter and Cosmology

AThe universe, as scientists have long understood it, consists of stars, planets, gases, and dust — matter that emits or reflects light and can therefore be detected by conventional telescopes. Nevertheless, decades of astronomical observation have revealed a profound discrepancy: the visible matter that astronomers can measure accounts for only approximately five percent of the total mass and energy of the universe. The remaining ninety-five percent appears to be composed of two mysterious components — dark matter and dark energy — whose precise nature has not yet been established. Given that this invisible material exerts measurable gravitational effects on galaxies and galaxy clusters, its existence has become one of the most compelling and widely accepted hypotheses in modern cosmology.

BDark matter, despite never having been directly observed, has been inferred through several independent lines of evidence. Perhaps the most convincing of these is the observed rotation of spiral galaxies. According to Newtonian mechanics, stars located at the outer edges of a galaxy should orbit its centre more slowly than those near the core, much as distant planets in our solar system travel more slowly than inner ones. In contrast, astronomical measurements have consistently shown that outer stars rotate at roughly the same speed as inner stars, a phenomenon that could only be explained if vast quantities of unseen matter were providing additional gravitational pull throughout the galaxy. Consequently, cosmologists have proposed that a diffuse halo of dark matter surrounds each galaxy, extending far beyond its visible boundaries.

CFurther evidence for dark matter has emerged from the study of gravitational lensing — a phenomenon in which the gravity of a massive object bends light passing near it, distorting images of background galaxies. If only visible matter were present, the degree of lensing observed around galaxy clusters would be far weaker than what astronomers have recorded. The fact that lensing effects are significantly stronger than visible mass alone can account for suggests that enormous quantities of invisible matter must be contributing to the total gravitational field. Additionally, computer simulations of cosmic structure formation, which model how galaxies and clusters have evolved over billions of years, produce results that match observed reality only when dark matter is included as a parameter.

DSome scientists have proposed alternative explanations that do not require dark matter at all. Modified Newtonian Dynamics, known as MOND, suggests that the laws of gravity themselves may behave differently at very low accelerations, which would account for galactic rotation curves without invoking invisible matter. While MOND has attracted some support, it has struggled to explain the full range of observational evidence, particularly the behaviour of galaxy clusters during collisions, such as the famous Bullet Cluster, where the gravitational centre of mass appears to have separated from the visible gas clouds. Consequently, the mainstream scientific community has continued to favour dark matter as the more comprehensive and consistent explanation, though the debate has not been fully resolved.

EDespite considerable theoretical progress, the fundamental question of what dark matter actually consists of remains unanswered. Several candidates have been proposed, including Weakly Interacting Massive Particles, commonly referred to as WIMPs, and axions — hypothetical subatomic particles predicted by certain extensions of standard particle physics. Experiments conducted in deep underground laboratories, designed to shield detectors from cosmic radiation, have so far failed to directly detect any dark matter particle. If such particles do exist, they interact so weakly with ordinary matter that current detection technologies may be insufficient to capture them. Nevertheless, ongoing advances in detector sensitivity and the continued operation of particle accelerators such as the Large Hadron Collider offer the scientific community genuine hope that the mystery of dark matter will eventually be resolved.