The universe is an astoundingly massive place. When we look up at the night sky, we can see thousands of stars, planets, galaxies and other celestial objects. But what makes up the vast majority of the universe is actually invisible to us – dark matter and dark energy. Together, these two mysterious components are estimated to make up a whopping 95% of the total mass-energy density of the universe. So what exactly are dark matter and dark energy and how do scientists know they exist if we can’t directly observe them? Let’s take a closer look.
What is dark matter?
Dark matter is an invisible substance that makes up about 25% of the universe. The existence of dark matter was first proposed in the 1930s when astronomers observed that galaxies and galaxy clusters behaved as if they had a lot more mass than could be accounted for by visible matter alone. The galaxies were rotating so rapidly that they should have flown apart if they contained only the mass that we can see. This led scientists to theorize that an invisible, mysterious substance, dubbed “dark matter,” must provide enough mass through gravity to hold these celestial structures together.
Evidence for dark matter
Since its proposal, various lines of evidence have shown that dark matter really does exist and is pervasive throughout the universe:
- Galaxy rotation curves – As mentioned earlier, the speeds at which galaxies rotate indicates there is more mass present than we can see.
- Gravitational lensing – Massive galaxy clusters can bend and distort the light from more distant galaxies through gravitational lensing, and this effect is larger than expected from just normal visible matter.
- Cosmic microwave background – Small fluctuations in the cosmic microwave background radiation provide clues about the amount of mass in the early universe, which includes dark matter.
- Galaxy cluster collisions – When galaxy clusters collide, the motion of the hot gas clouds indicates the presence of unseen dark matter.
- Big Bang nucleosynthesis – Calculations of how much normal matter was produced in the first few minutes after the Big Bang show that there must be more matter in the universe than just normal atoms.
So while we can’t directly see or touch dark matter, its gravitational influence can be observed throughout the universe.
What is dark matter made of?
The composition of dark matter remains one of the biggest mysteries in astrophysics today. Scientists aren’t sure what type of particle or object dark matter consists of, but they have some leading theories:
- WIMPs (Weakly Interacting Massive Particles) – Hypothetical heavy particles, with masses between 10 to 1000 times the mass of a proton, that interact only via gravity and weak nuclear force.
- Axions – Extremely light elementary particles with very low masses.
- MACHOs (Massive Compact Halo Objects) – Astronomical bodies such as black holes, neutron stars, brown dwarfs and rogue planets that don’t emit enough radiation to be detected.
- Sterile neutrinos – Hypothetical “sterile” particles that would interact only via gravity.
Experiments underway such as at the Large Hadron Collider and other detectors may provide clues to pin down the particle identity and properties of dark matter.
How much of the universe is dark matter?
According to the best current measurements, dark matter makes up about 25-27% of the total mass-energy density of the universe. This percentage has been deduced from multiple complementary observational techniques including cosmic microwave background measurements, galaxy surveys, gravitational lensing, and others.
While the exact figure has some uncertainty, around a quarter of the total cosmological pie seems to be dominated by this invisible substance. That makes dark matter nearly five times more prevalent than normal, visible matter, which constitutes only about 5% of the universe!
Dark Energy
What is dark energy?
Dark energy is an even more mysterious force that makes up about 70% of the universe. It is hypothesized to be behind the ongoing expansion of the universe that causes distant galaxies to accelerate away from us. Unlike normal matter and dark matter, dark energy has a negative pressure that counteracts gravity and drives this acceleration.
The existence of dark energy was inferred in the late 1990s when two independent teams studied the brightness of distant supernovae. They found that the supernovae were dimmer than expected, indicating that they were farther away than they should have been based on existing cosmological models. This meant that the expansion rate of the universe was not slowing down as expected, but accelerating instead due to an unknown energy pervading all of space.
Evidence for dark energy
Some key evidence that dark energy exists includes:
- Accelerating universe – Multiple probes now confirm that the expansion of the universe is speeding up over time due to dark energy’s repulsive gravity.
- Cosmic microwave background – Small fluctuations in this relic radiation contain signatures of dark energy’s influence in the early universe.
- Integrated Sachs-Wolfe effect – Dark energy’s effect on the growth of large scale structure can be detected in variations in the cosmic microwave background.
- Baryon acoustic oscillations – Echoes of early density variations that serve as a “standard ruler” to measure the expansion history of the universe.
- Redshift-space distortions – Mapping the growth rate of the large scale structure provides independent evidence for dark energy.
What is dark energy made of?
Dark energy is typically modeled in physics as the cosmological constant Λ, representing the energy density of the vacuum of space itself. However, its precise nature is still a major open question. Leading hypotheses include:
- Vacuum energy – The intrinsic, fundamental energy of the vacuum gives space-time an inherent tendency to expand.
- Quintessence – Dynamic field with negative pressure similar to the Higgs field but slowly evolving over time.
- Phantom energy – Exotic substance with equation of state where pressure is more negative than -1 times energy density.
- Modified gravity – Theories like f(R) gravity that modify general relativity to produce accelerated expansion without new energy.
Upcoming experiments and observations aim to distinguish between these possible explanations for dark energy.
How much of the universe is dark energy?
According to the most widely accepted cosmological model, the ΛCDM (Lambda Cold Dark Matter) model, dark energy constitutes about 70% of the total mass-energy budget of the universe. This value is derived from multiple independent measurements, including of the cosmic microwave background, the clustering of galaxies, distances to supernovae, and other observables.
So between dark matter and dark energy combined, a whopping 95% of the cosmos is comprised of stuff we cannot directly see or detect! Only about 5% is made of normal matter that makes up stars, planets, gas, dust, living beings, and the other visible contents of the universe. Dark matter and dark energy dominate the large-scale structure and evolution of our universe.
Conclusion
While dark matter and dark energy remain mysterious, they are firmly established as real and abundant components of our universe through rigorous astrophysical observations and models. Together, these invisible substances account for about 95% of the total mass-energy density of the universe and drive the growth and accelerated expansion of the cosmos. Dark matter provides extra gravitational glue that holds galaxies and galaxy clusters together, while dark energy exerts a repulsive force that overcomes gravity and causes the universe’s expansion to speed up.
Ongoing and future experiments seek to uncover the particle nature of dark matter and the precise properties of dark energy. This includes efforts at the Large Hadron Collider, searches for dark matter annihilation signals, baryon acoustic oscillation surveys, supernova observations, and more. Unraveling these mysteries could lead to revolutionary discoveries in fundamental physics that change our understanding of the universe’s origin, evolution, and ultimate fate. The cosmos’ biggest questions remain wide open.
| Component | Percentage of Total Density |
|---|---|
| Dark Energy | 70% |
| Dark Matter | 25% |
| Normal Matter | 5% |