Definition of Dark Matter
Dark matter is a hypothetical type of matter that does not interact with electromagnetic radiation, making it invisible to telescopes. It is inferred to exist because it exerts gravitational influence on visible matter, radiation, and the large-scale structure of the universe. Unlike ordinary matter, dark matter accounts for approximately 27% of the universe's mass-energy content, far outweighing the visible matter that forms stars, planets, and galaxies.
Key Properties and Components
Dark matter is non-baryonic, meaning it is not composed of protons and neutrons like ordinary matter. Its primary interaction is through gravity, with possible weak interactions but no significant electromagnetic ones. Candidates include weakly interacting massive particles (WIMPs) or axions, though its exact composition remains unknown. This matter is distributed in halos around galaxies, influencing their formation and stability without directly participating in atomic processes.
Evidence from Galaxy Rotation Curves
A key example of dark matter's evidence comes from observations of galaxy rotation curves. In the 1970s, astronomers like Vera Rubin measured how stars in spiral galaxies orbit the center. According to Newtonian gravity, stars farther from the center should orbit slower, but they maintain similar speeds, indicating unseen mass—dark matter—providing the extra gravity needed to prevent galaxies from flying apart. This flat rotation curve is observed in numerous galaxies, such as the Milky Way.
Broader Implications and Additional Evidence
Dark matter is crucial for understanding the universe's evolution, as it facilitated the clumping of matter after the Big Bang, leading to galaxy formation. Further evidence includes gravitational lensing, where light from distant objects bends more than expected due to dark matter's mass; the cosmic microwave background's temperature fluctuations, which match predictions from dark matter models; and the Bullet Cluster collision, where hot gas slowed but gravitational effects separated, revealing dark matter's independent distribution. These applications underscore dark matter's role in cosmology.