Dark matter is a hypothetical form of matter that is believed to make up a significant portion of the total mass in the universe. It is called "dark" because it does not interact with light or other forms of electromagnetic radiation, making it invisible or "dark" to our current observational methods.
The existence of dark matter was proposed to explain certain observations and phenomena in the universe that cannot be accounted for by the known forms of matter, such as stars, planets, and galaxies. These include the gravitational effects on the motion of galaxies, the distribution of matter on large scales, and the fluctuations observed in the cosmic microwave background radiation.
Dark matter is considered to be "outside" or separate from our visible universe in the sense that it does not interact with light and other electromagnetic forces, which are the primary means by which we observe and interact with ordinary matter. Dark matter is thought to be a different type of particle or matter that interacts only through gravity and possibly weak nuclear forces.
There are several proposed theories and models that attempt to explain the nature of dark matter. One widely accepted theory is the Cold Dark Matter (CDM) model, which suggests that dark matter consists of non-interacting particles that were created during the early stages of the universe. These particles would have formed structures known as dark matter halos, providing the gravitational scaffolding for the formation of galaxies and large-scale structures.
While dark matter is considered to be "outside" our visible universe in the sense of its lack of electromagnetic interactions, it is still an integral part of the overall universe. It is believed to permeate throughout space, influencing the distribution and behavior of ordinary matter through its gravitational effects.
It's important to note that the nature of dark matter is still a subject of ongoing scientific research and investigation. Numerous experiments and observations are being conducted to directly detect or indirectly study dark matter particles. Discovering the true nature of dark matter would provide valuable insights into the fundamental structure and evolution of the universe.