Dark matter

Dark matter is an invisible and hypothetical form of matter. It is a major mystery in astronomy and cosmology. Scientists cannot see it because it does not interact with light. It does not emit, absorb, or reflect electromagnetic radiation. We know it exists because of its gravitational effects. These effects cannot be explained by general relativity alone. Without dark matter, there would not be enough visible matter to explain how the universe works.

Dark matter acts as a gravitational scaffolding for the entire universe. After the Big Bang, dark matter clumped into large blobs. These blobs formed narrow filaments across space. These filaments created a cosmic web. Entire galaxies appear like tiny particles sitting on this web. Dark matter helps drive the formation and evolution of galaxies. It also influences the current structure of the observable universe.

We can observe dark matter through several specific mechanisms. One method is gravitational lensing. This occurs when the gravity of a massive object bends light from a distant source. Another way is by watching galaxy clusters. We can see how galaxies move within these large groups. Dark matter also affects the cosmic microwave background. These are tiny temperature variations left over from the early universe.

Scientists classify dark matter based on its velocity. They use terms like "cold," "warm," or "hot" dark matter. This classification depends on the free streaming length of the particles. Most recent models favor a cold dark matter scenario. In this scenario, cosmic structures emerge through the gradual accumulation of particles. This process helps build the large structures we see today.

The history of this discovery spans over a century. In 1884, Lord Kelvin suggested there might be many dark bodies near the Sun. In 1906, Henri Poincaré used the term "dark matter" to describe these ideas. In 1933, Fritz Zwicky studied the Coma Cluster at Caltech. He used the virial theorem to study galaxy motions. Zwicky found that the cluster had 400 times more mass than was visible. He concluded that unseen mass provided the necessary gravitational attraction.

In the 1970s, the hypothesis of dark matter became widely accepted. Astronomers like Vera Rubin and Kent Ford studied galaxy rotation curves. They used new spectrographs to measure how spiral galaxies rotate. They found that galaxies rotate much faster than visible stars should allow. This suggested that galaxies are surrounded by large halos of dark matter. Other researchers used radio astronomy to map hydrogen gas. This provided even more evidence of these invisible halos.

The composition of the universe is dominated by these invisible components. Ordinary baryonic matter makes up only 5% of the mass-energy content. Dark matter accounts for 26.8% of the total content. The remaining 68.2% is a form of energy called dark energy. This means dark matter represents 85% of the total mass in the universe. However, dark matter is very sparse in our local area. The total dark matter out to Neptune's orbit is only about 10^17 kg. This is roughly the mass of a large asteroid.

Researchers are currently searching for the identity of dark matter particles. The most popular theory involves weakly interacting massive particles, or WIMPs. Another strong candidate is the axion, a very light particle. Some scientists even suggest dark matter could be primordial black holes. Large experiments like LZ use liquid xenon to try to detect WIMPs. So far, these direct detection experiments have only reported null results. This has caused some scientists to look closer at alternative theories.

Some astrophysicists propose that we do not need dark matter at all. They argue we might need to modify the laws of gravity. These theories include modified Newtonian dynamics, known as MOND. Other ideas include tensor-vector-scalar gravity and entropic gravity. However, these modified gravity theories struggle to explain all observations. Most evidence suggests that some form of dark matter is still required. The search for the truth continues through new technology and observation.