What do we know about dark matter?

Currently, dark matter is a theoretical construct, believed to make up as much as 90% of the universe's mass. Despite extensive research, it has yet to be directly observed. The challenge lies in the predictions that dark matter particles interact very weakly, both with regular matter and amongst themselves.

Source: apod.nasa.gov

Numerous experiments are underway to detect dark matter particles, but their results remain inconclusive.

Section 1.1: Why Do Scientists Believe Dark Matter Exists?

Gravity is one of the most reliable forces we know. With the principles of celestial mechanics, we can accurately predict the trajectories of celestial bodies. Given sufficient data about the entities in a system, we can forecast their movements over time.

When observed motions deviate from predictions, it indicates the presence of additional gravitational influences. This principle led to the discovery of Neptune and Sirius B. Similarly, discrepancies in Mercury's orbit provided evidence supporting Einstein's theory of relativity.

In their studies of galaxy rotation, astronomers found that stars at the edges of galaxies rotate at speeds comparable to those near the centers, despite calculations suggesting they should be much slower. This observation led to the hypothesis that galaxies contain significantly more mass than can be accounted for by visible stars, hinting at the existence of dark matter. Essentially, dark matter serves to clarify the gravitational anomalies observed in galaxies.

Section 1.2: Evidence Supporting Dark Matter

One of the most compelling pieces of evidence for dark matter is derived from observations of the Bullet galaxy cluster.

The Bullet galaxy cluster is formed from two galaxy clusters that have recently collided.

The visible matter, primarily composed of intergalactic gas, appears in pink in X-ray observations, while the inferred dark matter, responsible for gravitational lensing effects, is depicted in blue.

During the collision, the galaxies and surrounding dark matter passed through one another, while the clouds of intergalactic gas collided, resulting in heating and slowing.

The gravitational lensing effect illustrates the bending of light around massive objects. The presence of dark matter is inferred from the gravitational lensing observed in the blue regions of the Bullet cluster, which demonstrates a significant mass concentration. If dark matter did not exist, we would expect the lensing effect to be strongest in areas with heated gas. However, the evidence indicates that this effect is most pronounced where dark matter and normal matter are spatially separated due to the collision.

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