Dark matter refers to an unknown substance that scientists believe makes up a significant portion of the total mass of the universe. This substance does not interact with light or any other form of electromagnetic radiation that we can observe, which makes it difficult to detect or study directly. However, its presence can be inferred through gravitational effects on visible matter, such as galaxies and cosmic microwave background radiation. The exact nature of dark matter remains a topic of debate and investigation in the field of astronomy and physics.