The argument for dark matter as opposed to an alternate theory of gravity is primarily based on observational evidence and its consistency with our current understanding of the universe. Here are some key points in favor of dark matter:
Gravitational Effects: Dark matter is invoked to explain observed gravitational effects that cannot be accounted for by the visible matter in galaxies and galaxy clusters. For example, the rotational velocities of stars within galaxies are observed to be much higher than what would be expected based on the visible matter alone. Dark matter provides the additional gravitational pull necessary to explain these observations.
Cosmological Observations: Dark matter is also supported by observations of the large-scale structure of the universe. The distribution of galaxies and galaxy clusters, as seen in cosmic microwave background radiation, gravitational lensing, and galaxy surveys, indicates the presence of more matter than what we can detect through electromagnetic radiation. This additional matter is attributed to dark matter.
Bullet Cluster Observations: The Bullet Cluster is a famous example often cited in support of dark matter. It is a merging galaxy cluster where the visible matter (gas and galaxies) and the gravitational lensing of the matter can be separated. The observations show that the mass inferred from gravitational lensing is offset from the visible matter, providing evidence for the existence of dark matter.
Cosmic Microwave Background (CMB): Precise measurements of the cosmic microwave background radiation support the presence of dark matter. The patterns of temperature fluctuations in the CMB are consistent with the predictions of a universe containing dark matter, as revealed through computer simulations and theoretical models.
On the other hand, proponents of alternative theories of gravity argue that modifying our understanding of gravity itself can account for the observed phenomena without invoking dark matter. These modified theories of gravity, such as Modified Newtonian Dynamics (MOND) and Modified Gravity (MOG), propose that our current understanding of gravity breaks down at certain scales and that it can be modified to explain the observed phenomena.
The arguments for alternative theories of gravity include:
Explaining Galactic Rotation Curves: Modified gravity theories aim to explain the observed high rotational velocities of stars in galaxies without the need for additional invisible matter. They propose modifications to the laws of gravity that can reproduce the observed rotation curves without invoking dark matter.
Simplicity and Parsimony: Alternative theories of gravity argue that adding an entirely new form of matter (dark matter) to the universe is a complex and unnecessary explanation. They propose that modifying the laws of gravity itself is a more elegant and parsimonious approach.
Success at Small Scales: Some modified gravity theories have been successful in reproducing the observed dynamics of galaxies and galaxy clusters without dark matter. They claim that these successes demonstrate the viability of alternative theories without the need for an unknown and elusive dark matter particle.
It's important to note that the debate between dark matter and alternative theories of gravity is ongoing, and scientists continue to explore and test both possibilities through further observations, simulations, and experiments to better understand the nature of the universe.