What Is Dark Matter?

Dark matter is a substance that makes up a significant portion of the universe. It is called “dark” because it does not interact with light or other forms of electromagnetic radiation. This means that we cannot detect it directly, but only through its gravitational effects on visible matter. Dark matter is estimated to make up about 27% of the total matter in the universe, with the rest being ordinary matter and dark energy.

The existence of dark matter was first proposed in the 1930s by Swiss astronomer Fritz Zwicky, who noticed that the mass of some galaxy clusters was much greater than what could be accounted for by visible matter alone. Since then, many other observations have confirmed the presence of dark matter in the universe.

Observations of Dark Matter

One of the most compelling pieces of evidence for dark matter comes from the observation of gravitational lensing. This occurs when the gravitational field of a massive object, such as a galaxy, bends the path of light passing near it. By observing the way that light is bent, astronomers can infer the presence and distribution of the invisible matter that is causing the gravitational distortion.

Another way that dark matter is observed is through the motion of stars and gas within galaxies. If a galaxy contains more mass than can be accounted for by visible matter, then the stars and gas should move faster than they would under the influence of visible matter alone. This is indeed what has been observed in many galaxies, providing further evidence for the presence of dark matter.

Theories of Dark Matter

Despite its pervasive presence in the universe, the nature of dark matter remains a mystery. There are several theories that attempt to explain what dark matter might be made of, but so far none of them have been definitively proven or disproven.

One popular theory is that dark matter is composed of weakly interacting massive particles (WIMPs). These particles would interact with regular matter only through the weak nuclear force, making them difficult to detect. Other theories propose that dark matter is made up of primordial black holes, or that it is a manifestation of extra dimensions in space.

Example of Dark Matter in Action

One of the most striking examples of the role of dark matter in the universe is the phenomenon of galaxy rotation curves. When astronomers measure the rotation speeds of stars and gas in a spiral galaxy, they find that the outer parts of the galaxy are rotating just as fast as the inner parts. This is unexpected, since if the galaxy were composed only of visible matter, the outer parts should be rotating more slowly due to the weaker gravitational force. The most plausible explanation for this is that the galaxy contains a large amount of dark matter, which provides the extra gravitational pull needed to keep the outer parts rotating at the same speed as the inner parts.