The observed speed of galaxies spinning in the cosmos presents one of the most profound unresolved questions in modern astrophysics. Astronomers have measured the rotational velocity of galaxies, expecting the movement to align with the gravitational pull of all the visible stars, gas, and dust, based on established laws of gravity. The discrepancy arises because galaxies consistently rotate much faster than they should, based on the amount of luminous material they contain. This phenomenon, known as the galaxy rotation anomaly, suggests that a significant portion of a galaxy’s mass remains unseen. The interpretation of this fast rotation has led to two competing hypotheses: the existence of a mysterious, invisible substance that provides the missing gravity, or a fundamental modification to how gravity operates on galactic scales.
Understanding the Galaxy Rotation Anomaly
Astronomers determine a galaxy’s rotation speed by analyzing the light emitted from its stars and gas using the Doppler shift technique. Light from objects moving away from Earth is stretched toward the red end of the spectrum, while light from objects moving toward us is compressed toward the blue end. By mapping these shifts across the galactic disk, a rotation curve is constructed, which plots orbital velocity against the distance from the galaxy’s center.
Based on the physics describing our solar system, the rotation curve was expected to resemble the motion of planets around the sun. Since most of a galaxy’s visible mass is concentrated in the central bulge, Newtonian gravity predicts that the orbital speed of stars and gas clouds should sharply decrease farther out. However, observations of numerous spiral galaxies revealed a different pattern entirely.
Instead of decreasing, the rotation speeds remain remarkably flat or nearly constant, extending far beyond the region where visible matter trails off. This uniform high speed implies that the gravitational force acting on the outer regions of a galaxy is much stronger than the visible mass can account for. The stars on the outskirts are orbiting at speeds that should fling them into intergalactic space, yet they remain gravitationally bound. This indicates that a substantial, non-luminous mass must be distributed throughout the galaxy’s outer halo.
The Dark Matter Hypothesis
The most widely accepted interpretation is the Dark Matter Hypothesis, which posits that galaxies are embedded within a vast, spherical halo of non-luminous mass. This unseen material provides the extra gravitational pull needed to keep the outer stars orbiting at their high, observed velocities without requiring a change to the laws of gravity. Dark matter does not emit, absorb, or reflect light, making it electromagnetically invisible and impossible to detect directly using conventional telescopes.
The substance is considered non-baryonic, meaning it is not composed of protons and neutrons, which constitute all ordinary matter. This distinction is necessary because the abundance of ordinary matter is tightly constrained by observations of Big Bang nucleosynthesis and the cosmic microwave background. Dark matter is thought to interact with ordinary matter only through gravity and possibly the weak nuclear force.
The leading candidate for this substance is the Weakly Interacting Massive Particle (WIMP). WIMPs are hypothesized to be heavy, slow-moving particles, classifying them as “cold dark matter.” Their slow movement was necessary for them to clump together in the early universe, forming the gravitational scaffolding upon which galaxies later formed. Cosmological models suggest that dark matter accounts for approximately 85% of the total mass in the universe, dominating the gravitational dynamics of galaxies and galaxy clusters.
Alternative Gravitational Theories
A competing interpretation argues that the problem does not lie with missing mass, but rather with an incomplete understanding of gravity itself at certain scales. This approach seeks to modify the laws of physics instead of inventing a new particle. The most prominent of these alternatives is Modified Newtonian Dynamics (MOND), first proposed by physicist Mordehai Milgrom in 1983.
MOND suggests that Newton’s law of gravity, which works flawlessly in high-acceleration environments like the solar system, breaks down in the low-acceleration regimes found in the outer edges of galaxies. When the acceleration generated by a galaxy’s mass drops below a certain universal value, known as the acceleration scale, the gravitational force becomes stronger than predicted by Newtonian physics.
This modification naturally explains the flat rotation curves without the need for a dark matter halo. According to MOND, the gravitational influence of the visible baryonic matter is enhanced when the gravitational field is weak, providing the necessary extra pull to keep the outer stars in orbit. This modification offers a direct mathematical relationship between the observable baryonic mass of a galaxy and its rotation curve, which MOND proponents argue is predicted with remarkable accuracy.
Distinguishing Between Interpretations
The scientific community actively seeks observations that can definitively distinguish between the Dark Matter and Modified Gravity interpretations. The most compelling evidence favoring the Dark Matter hypothesis comes from the observation of the Bullet Cluster. This structure resulted from a collision between two massive galaxy clusters, which effectively separated the ordinary matter from the unseen mass.
During the collision, the hot, X-ray-emitting gas—which constitutes the majority of the ordinary matter—slowed down and collected in the center due to electromagnetic drag. However, gravitational lensing observations showed that the bulk of the mass, which dictates the gravitational field, continued to pass through the collision largely unimpeded, separating spatially from the gas. Since MOND modifies how gravity acts upon ordinary matter, it struggles to explain this physical separation.
Observations of the Cosmic Microwave Background (CMB), the faint afterglow of the Big Bang, provide strong support for the Cold Dark Matter model (Lambda-CDM). The precise structure of the temperature fluctuations in the CMB is well-fitted by the Dark Matter model, a result that modified gravity theories have difficulty reproducing. Experimental efforts continue with underground detectors attempting to directly observe a WIMP particle colliding with an atomic nucleus, an interaction predicted by the Dark Matter hypothesis.