The Contents of the Universe: Ordinary Matter, Dark Matter, and Dark Energy
What is the universe made of?
Three major components
Modern cosmology says the universe contains ordinary matter, dark matter, and dark energy. Ordinary matter makes up stars, planets, gas, and people. Dark matter does not emit or absorb light in the usual way, but it affects gravity. Dark energy is the name given to whatever seems to be driving the accelerated expansion of the universe.
Three components of the cosmos
Ordinary matter
- Builds atoms and familiar objects
- Emits and absorbs light
- A small fraction of the universe by mass-energy
Dark matter
- Seen through gravitational effects
- Does not appear to shine like ordinary matter
- Helps hold galaxies and clusters together
Key idea
The universe appears to be dominated by components we cannot directly see with ordinary light.
Evidence for dark matter
Galaxies rotate in ways that suggest there is more mass than we can see. Galaxy clusters and gravitational lensing also point to extra unseen mass. Scientists call this unseen component dark matter.
Which component is associated with accelerated cosmic expansion?
Dark energy is the name for the cause of the observed acceleration.
Correct answer: Dark energy
Which type of matter makes up stars and people?
Ordinary matter is also called baryonic matter.
Correct answer: Ordinary matter
Why these components matter
Different components affect the universe in different ways. Matter tends to clump under gravity, while dark energy affects the large-scale expansion. Understanding their balance is central to modern cosmology.
Why do astronomers think dark matter exists?
We infer dark matter from its gravitational influence on visible objects and light.
Correct answer: Because gravitational effects are stronger than visible matter alone explains
What is one reason dark energy is difficult to study?
We observe its effect on expansion, but we do not yet know exactly what it is.
Correct answer: Its physical nature is still unknown