dark matter

History

The first to speak of 'dark matter' was Swiss astronomer Fritz Zwicky in 1933. Studying the motion of galaxies in the Coma Cluster, he found that their velocities were too high for the observed mass and suggested the presence of hidden 'dark' mass. In the 1970s, American astronomer Vera Rubin, studying spiral galaxies, obtained convincing evidence: rotation curves remained flat at large distances from the center, indicating a massive invisible halo. These works made dark matter a central problem in cosmology.

How it works

Imagine a snowball with a stone at its center. When the snowball rolls, the snow flies apart, but the stone remains. Dark matter plays the role of gravitational 'glue': it does not interact with light, but its gravity holds stars in galaxies, preventing them from flying apart due to rapid rotation. Exactly how it 'works' we don't know: candidates range from massive but weakly interacting particles (WIMPs) to axions or even primordial black holes.

History

The first to speak of 'dark matter' was Swiss astronomer Fritz Zwicky in 1933. Studying the motion of galaxies in the Coma Cluster, he found that their velocities were too high for the observed mass and suggested the presence of hidden 'dark' mass. In the 1970s, American astronomer Vera Rubin, studying spiral galaxies, obtained convincing evidence: rotation curves remained flat at large distances from the center, indicating a massive invisible halo. These works made dark matter a central problem in cosmology.

How it works

Imagine a snowball with a stone at its center. When the snowball rolls, the snow flies apart, but the stone remains. Dark matter plays the role of gravitational 'glue': it does not interact with light, but its gravity holds stars in galaxies, preventing them from flying apart due to rapid rotation. Exactly how it 'works' we don't know: candidates range from massive but weakly interacting particles (WIMPs) to axions or even primordial black holes.

History

In 1933, Fritz Zwicky applied the virial theorem to the Coma Cluster and calculated that much more mass than visible was required to hold the galaxies; he coined the term 'dark matter'. In the 1970s, Vera Rubin and Kent Ford, using a spectrograph to measure Doppler shifts, constructed rotation curves of spiral galaxies and showed that at the outskirts, velocities remain constant, which contradicts Newton's law of gravitation given the visible mass distribution — a huge invisible halo was required. Subsequent observations of gravitational lensing (e.g., the 'Bullet Cluster' — a collision of clusters) and the cosmic microwave background confirmed the existence of dark matter as the dominant non-baryonic component. Theoretical predictions within supersymmetry actively stimulated searches for WIMP particles in underground detectors.

How it works

Dark matter interacts with ordinary matter through gravity, which is described by Newton's law of universal gravitation and Einstein's equations. Galaxy rotation curves are explained by assuming that mass is concentrated in an extended halo whose density falls off as ρ ~ r^{-2}. Gravitational lensing, predicted by general relativity, allows mapping the distribution of dark matter, for example, by the distortion of shapes of distant galaxies. The cosmological theory of structure formation is based on primordial density fluctuations; dark matter, being cold (CDM), clumps earlier than ordinary matter, creating gravitational 'wells' into which baryonic matter subsequently falls. The anisotropy of the cosmic microwave background encodes the density ratios of the universe's components. Non-relativistic equations of motion (Newton's law) are used to describe dynamics in halos. A key problem is the lack of detection of hypothetical particles on Earth.

Open problems & fun facts

Identification of dark matter particles: what physical nature do they have?; Discrepancies between simulations of dark matter distribution in the ΛCDM model and observations of dwarf galaxies (cusp problem and satellite deficit).; Possible modification of gravity instead of introducing dark matter (MOND-like theories).; The role of dark matter in the evolution of the first stars and galaxies.

💡 The most popular hypothesis is that dark matter consists of WIMPs, particles that pass through us every second by the millions, but we don't feel them. And there are galaxies that are almost completely devoid of dark matter, such as DF2 and DF4.