Universe Composition, Dark Matter, and Cosmic Structures

Universe Composition

The universe is immense and comprises millions of bodies. Not everything in the universe can be observed, so the universe currently consists of an observable part. We know that there are billions of galaxies. Each galaxy contains thousands of stars, nebulae, and planets.

From a chemical point of view, the observable universe is described as being made up of: 75% hydrogen, 20% helium, and 5% other elements. Isaac Newton discovered that when sunlight passes through a prism, it separates into bands of colors (the light spectrum). We observe some black lines within this spectrum. In the laboratory, if we pass white light through containers with hydrogen and helium gas, these elements absorb specific colors of light, resulting in black lines appearing in the spectrum. By comparing the black lines in the sun's spectrum with those produced by hydrogen and helium in the lab, we conclude that the sun's outer layers are primarily composed of hydrogen and helium.

Dark Matter: The Unobserved Universe

Today, it is accepted that the observed gravitational attraction affecting gas and dust is greater than what can be accounted for by observable matter alone. This implies the existence of a type of unobserved matter, known as dark matter, which emits no radiation and therefore cannot be directly detected. It is estimated that only about 10% of all matter in the universe is observable, with the remaining 90% being dark matter. Evidence for dark matter includes the rotation speeds of galaxies and the gravitational interactions observed between two galaxies, which cannot be explained by visible matter alone. We cannot see dark matter directly, but we observe its gravitational effects.

Large Structures of the Universe

Galaxies

Galaxies are vast systems containing stars, gas, and dust, bound together by gravity. They differ in shape, size, and the number of stars they contain. Based on their shape, galaxies can be classified into:

• Spiral Galaxies: Contain a central core (or bulge) and spiral arms. Our Milky Way is a spiral galaxy.
• Elliptical Galaxies: They lack distinct arms and have a generally smooth, ovoid shape. They often consist of older stars and can be among the largest galaxies observed.
• Irregular Galaxies: These have no definite shape and are among the most abundant types. Some smaller ones are classified as dwarf galaxies.

Galaxy Clusters

Galaxies often group together to form larger structures called clusters. Observations show that elliptical galaxies tend to be found near the center of clusters, while spiral and irregular galaxies are often distributed around the periphery. Our Milky Way is part of a cluster called the Local Group, which includes other spiral galaxies like Andromeda. The Local Group, in turn, belongs to a larger structure called the Virgo Supercluster.

Nebulae

Nebulae are interstellar clouds of gas and dust, appearing in various forms. Different types can be distinguished:

• Emission Nebulae: These nebulae emit their own light (electromagnetic radiation). They are often associated with the formation of new stars.
• Reflection Nebulae: These are large dust clouds illuminated by nearby stars. They reflect light but do not emit significant radiation themselves.
• Planetary Nebulae: These are shells of gas and dust ejected by stars nearing the end of their life cycle.
• Supernova Remnants: These are the expanding clouds of debris resulting from massive explosions (supernovae) that occur at the end of the life cycle of very large, massive stars.