How do scientists know that dark matter exists?
Scientists know that dark matter exists based on several lines of evidence, including:
1. Galactic rotation curves: The observed rotation curves of galaxies indicate that there is more mass present in galaxies than what can be accounted for by the visible matter (stars, gas, and dust). Dark matter provides an explanation for this observed discrepancy, as its gravitational pull contributes to the overall mass distribution.
2. Gravitational lensing: The phenomenon of gravitational lensing occurs when the path of light from distant galaxies is bent due to the gravitational influence of intervening mass. Gravitational lensing observations reveal that there is more mass present in the universe than can be attributed to visible matter, implying the existence of dark matter.
3. Large-scale structure formation: The observed distribution of galaxies in the universe, as well as the patterns seen in the cosmic microwave background radiation, suggest that the growth of large-scale structures (galaxy clusters, superclusters, etc.) required the presence of additional gravitational influence, namely dark matter.
4. Particle physics theories: Various particle physics theories predict the existence of particles beyond those in the Standard Model. Weakly Interacting Massive Particles (WIMPs) are among the proposed candidates for dark matter. While direct detection of dark matter particles is challenging, several experiments are underway aiming to detect them indirectly, corroborating their existence.
5. Collisions of galaxy clusters: The observation of collisions between galaxy clusters provides additional evidence for dark matter. When clusters collide, the visible matter interacts with each other, while the dark matter continues moving forward unaffected by such interactions, resulting in an offset between the visible and dark matter distributions.
Overall, a combination of these and other observational and theoretical pieces of evidence has led to the widely accepted consensus among scientists that dark matter exists. However, its precise nature and the identity of its constituent particles remain an area of active research and investigation.