What is Dark Matter?
Dark matter is a hypothetical form of matter that scientists believe makes up about 27% of the universe's mass-energy content. Unlike ordinary matter (like atoms, stars, and planets), dark matter does not interact with light or other electromagnetic radiation, making it invisible and undetectable by conventional telescopes. Its presence is inferred through its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.
Key Characteristics and Evidence
Dark matter's primary characteristic is its gravitational influence without electromagnetic interaction. Evidence for its existence comes from observations such as the rotational speeds of galaxies, where stars at the edges orbit too quickly to be held by the visible matter alone, and gravitational lensing, where light from distant objects bends more than expected around galaxy clusters. The cosmic microwave background radiation also shows patterns consistent with a significant amount of dark matter influencing the early universe's structure.
A Practical Example: Galaxy Rotation Curves
Imagine a merry-go-round where people on the outer edge spin slower than those closer to the center. For galaxies, based on visible matter, stars farther from the galactic center should orbit slower. However, observations show that stars on the outer fringes of galaxies often orbit at roughly the same speed as stars closer to the center. This unexpected uniform speed suggests there's an unseen, massive halo of dark matter encompassing the galaxy, providing extra gravitational pull to keep the outer stars moving fast.
Importance in Cosmology
Dark matter is crucial for understanding the formation and evolution of the universe. Without its gravitational scaffolding, galaxies and galaxy clusters would not have formed as they appear today, or at all. It provides the extra mass needed for cosmic structures to develop and holds them together, playing a fundamental role in the standard model of cosmology alongside dark energy and ordinary matter.