Whenever it comes to the inexplicable things existing in our universe, the controversy about the existence of dark matter always tops the list. No doubt, that even the most sensitive detectors have failed to find a convincing evidence to prove the existence of dark matter, still there are many unusual phenomenons happening in the cosmos that cannot be explained without considering the presence of this untraceable form of matter, and these observations have always kept the hope of directly detecting the dark matter in near future alive. Here, we have compiled a list of five most important phenomenons that truly support the fact that dark matter should exist!
Galaxy Rotation Curves
For Spiral galaxies like the Milky Way, we derive the gravitational mass by observing the motions of stars and gas clouds in the disk as they orbit the center. The rotation curve of a galaxy shows how the velocity of stars around the center varies as the distance from the center increases. From Kepler’s Second Law, it is expected that the rotation velocities will decrease with distance from the center, similar to the Solar System. However, this is not observed. Instead, most spiral galaxies show flat rotation curves out as far as we can trace them, even where no more stars are visible.
Therefore, to support the Kepler’s laws and to resolve this discrepancy, we conclude that the gravitational mass is more than 10 times more massive than the luminous mass, hence there should exist the non luminous matter, i.e. dark matter to compensate for this extra gravitational mass.
The Swiss astronomer Fritz Zwicky used the velocity dispersion of galaxies in clusters as determined from Doppler shifts to estimate their dynamical mass. However, with some exceptions, velocity dispersion estimates of elliptical galaxies do not match with the predicted velocity dispersion from the observed mass distribution, even by assuming complicated distributions of stellar orbits.
Again, the most obvious way to resolve this discrepancy comes by postulating the existence of the non-luminous matter.
Structure formation refers to the period after the Big Bang when density perturbations collapsed to form stars, galaxies, and clusters. Prior to the structure formation, the Friedmann solutions to general relativity described a homogeneous universe. Later, small anisotropies gradually grew and condensed the homogeneous universe into stars, galaxies and larger structures. Since ordinary matter is affected by radiation, its density perturbations are washed out and unable to condense into structure. If there were only ordinary matter in the universe, then there would not have been enough time for density perturbations to grow into the galaxies and clusters currently seen.
Hence, here dark matter comes to rescue and provides a solution to this problem as it is unaffected by radiation
Consisting of two colliding clusters of galaxies, the Bullet cluster is claimed to provide the best current evidence for the nature of dark matter. The bullet cluster provides a challenge for modified gravity theories because its apparent center of mass is far displaced from the baryonic center of mass.
However, this observation can be easily explained by the existing Standard dark matter theory. Hence, if dark matter does not exist, then it points out that the prevailing theory of gravity, i.e. general relativity is incorrect, which is not the case!
Cosmic Microwave Background (CMB)
Dark matter does not interact directly with radiation, but it does affect the CMB by its gravitational potential (mainly on large scales), and by its effects on the density and velocity of ordinary matter. The cosmic microwave background contains very small temperature anisotropies of a few parts in 100,000. A sky map of anisotropies can be decomposed into an angular power spectrum, which is observed to contain a series of acoustic peaks at near-equal spacing but different heights. These peaks have indicated that the universe contained about five times as much dark matter as normal matter when the neutral hydrogen formed. Combined with measurements of supernovae and the clustering of galaxies, this indicates that dark energy comprises 73 percent of the universe, dark matter 23 percent, and normal matter just 4 percent.
Along with these observational evidences, even the masses of galactic clusters collected by techniques of gravitational lensing, X-Ray energy spectrum, etc. are in reasonable agreement that dark matter outweighs visible matter by approximately 5 to 1. Hence, not finding dark matter in an experiment is not the evidence that it doesn’t exist. These indirect evidences are enough to keep the fire to hunt for the invisible burning. And, we hope to find at least one direct evidence of dark matter in our universe in near future.