Dark matter is a type of matter that is thought to make up a significant portion of the matter in the universe. It is called "dark" matter because it does not interact with light or other forms of electromagnetic radiation in any noticeable way, making it difficult to detect directly.
Dark matter is thought to exist because there is evidence that there is more matter in the universe than what can be observed directly. For example, the rotation of galaxies and the motion of galaxies within galaxy clusters indicates that there must be more mass present than what can be seen in the form of stars, gas, and dust. This missing mass is thought to be made up of dark matter.
Despite its name, dark matter is not necessarily "dark" in the sense of being invisible or imperceptible. It is called dark matter because it does not emit, absorb, or reflect light, making it difficult to detect directly. Scientists are still trying to understand the properties of dark matter and how it behaves, but it is thought to be a type of matter that does not interact with normal matter through any of the fundamental forces except gravity.
It is currently unknown what dark matter is made of, but there are several theories about its composition. Some scientists believe that dark matter could be made up of exotic particles that have not yet been discovered, while others think it could be made up of more familiar particles that simply do not interact with normal matter in the same way.