The biggest mystery of astrophysics. Dark matter and dark energy are two phenomena in astrophysics that cannot be directly observed, but their existence can be inferred through their effects on other visible matter and energy in the universe. Dark matter is a hypothetical form of matter that does not emit, absorb, or reflect light, and therefore cannot be detected by telescopes or other instruments that rely on detecting electromagnetic radiation. However, its gravitational effects can be observed on visible matter, such as stars and galaxies. The existence of dark matter was proposed to explain the observed gravitational effects on visible matter, which could not be accounted for by the visible matter alone. Dark energy is also a hypothetical form of energy that is believed to be responsible for the accelerating expansion of the universe. Like dark matter, it cannot be directly observed, but its effects can be inferred from observations of distant supernovae, cosmic microwave background radiation, and galaxy clustering. Unlike dark matter, dark energy is believed to have a repulsive effect on matter, causing it to move away from each other faster over time. Despite extensive research, the nature of dark matter and dark energy remains unknown, and they remain two of the biggest mysteries in modern astrophysics.