Final answer:
Dark matter is a non-baryonic component of the universe that cannot be directly observed. It plays a dominant role in galaxy formation and early evolution through its gravitational force. Evidence for dark matter is inferred from phenomena such as gravitational lensing and measurements of galactic rotation and extragalactic distances.
Step-by-step explanation:
The Nature of Dark Matter in Galaxies
Dark matter is a component of the universe that cannot be directly observed in any part of the electromagnetic spectrum. It plays a dominant role in galaxy formation and early evolution through its gravitational force. Scientists are still unsure about the exact nature of dark matter, but it is likely to be non-baryonic in nature, meaning it is not composed of normal matter like protons and neutrons.
Evidence for Dark Matter
The presence of dark matter in the universe has been inferred through various observations. For example, gravitational lensing can occur when light from a distant galaxy is bent by the gravitational field of dark matter, resulting in multiple images of the same galaxy. In addition, measurements of galactic rotation and extragalactic distances support the existence of dark matter.
Limitations of Baryonic Matter
Constraints on baryonic matter, which is composed of protons and neutrons, restrict how much of it can exist. If dark matter consisted of a large number of stellar black holes or old neutron stars, interstellar matter falling onto them would produce more X-rays than are observed. Furthermore, the formation of such objects would lead to a substantial loss of mass, resulting in higher abundances of heavy elements in young stars than what is observed.