Dark matter is a mysterious, invisible substance that makes up a significant portion of the universe's mass and appears to have gravitational effects on visible matter, such as galaxies and galaxy clusters. Although we cannot directly observe dark matter because it doesn't emit, absorb, or reflect light, its presence is inferred from its gravitational influence on visible matter.
Dark matter affects the universe in several ways:
1. Gravitational Binding: Dark matter's gravitational pull plays a crucial role in holding galaxies and galaxy clusters together. Without dark matter, galaxies would not have enough mass to explain their observed rotational speeds and would fly apart.
2. Cosmic Structure Formation: Dark matter is thought to have played a central role in the formation of cosmic structures, like galaxies and galaxy clusters. Its gravitational attraction helped gather gas and dust, leading to the formation of galaxies and stars.
3. Cosmic Microwave Background: Dark matter's presence has left an imprint on the cosmic microwave background (CMB), a faint radiation left over from the Big Bang. It contributed to the initial fluctuations in density that later led to the formation of galaxies and large-scale cosmic structure.
4. Gravitational Lensing: Dark matter can bend and distort the path of light as it travels through the universe, a phenomenon known as gravitational lensing. This effect is used by astronomers to map the distribution of dark matter in galaxy clusters.
5. Expansion of the Universe: Dark matter's gravitational pull affects the expansion rate of the universe. It counteracts the expansion caused by dark energy, a mysterious force driving the universe's accelerated expansion, resulting in a complex interplay between these two components.
Despite its significant influence, the true nature of dark matter remains unknown. It does not consist of ordinary matter particles like protons and electrons. Various theories and experiments are ongoing to better understand its properties and composition.