The rotation curves of galaxies show that the outer regions of galaxies rotate faster than expected based on the visible mass of the galaxy. This suggests that there is a large amount of unseen mass, or dark matter, in galaxies.

According to the latest estimates, dark matter makes up about 27% of the total energy density of the universe, while ordinary matter makes up only about 5%.

Weakly interacting massive particles (WIMPs) are hypothetical particles that are massive but interact very weakly with ordinary matter. They are one of the leading candidates for dark matter because they can explain the observed properties of dark matter, such as its abundance and distribution.

Direct detection experiments attempt to detect dark matter particles directly by measuring the energy they deposit in a detector material. This is a very challenging experiment, as dark matter particles are expected to interact very weakly with ordinary matter.

PandaX-4T is the largest direct detection experiment for dark matter, located in China. It uses a liquid xenon detector with a mass of 4 tons.

Gamma-ray astronomy is the most promising indirect detection method for dark matter because dark matter particles can annihilate each other and produce gamma rays. Gamma-ray telescopes can be used to search for these gamma rays, which would provide evidence for the existence of dark matter.

The Galactic Center excess is a significant gamma-ray excess that has been observed in the center of the Milky Way galaxy. This excess could be due to dark matter annihilation, but it could also be due to other astrophysical sources.

The future of dark matter research lies in a combination of more sensitive direct detection experiments, more powerful indirect detection experiments, and new theoretical models of dark matter. All of these approaches are necessary to solve the mystery of dark matter.

The presence of dark matter affects the expansion of the universe. Dark matter is thought to be a major contributor to the acceleration of the expansion of the universe, which means that the universe will eventually expand forever.

The most important question that remains to be answered about dark matter is its nature. What is dark matter made of? What are its properties? This is a fundamental question that has yet to be answered.