- Roughly 95% of the universe is dark matter and dark energy, both unexplained.
- Three independent experiments now suggest dark energy is weakening over time.
- What happened before the Big Bang remains an open question in physics.
Cosmology asks the largest questions science can pose. How did the universe begin? What is it made of? How will it end? These questions have moved from philosophy to precision science within a single century, driven by telescopes that can see back to the first light ever emitted and detectors that measure ripples in spacetime itself.
Yet the more precisely cosmologists measure the universe, the stranger it looks. Roughly 95% of its contents are invisible. The expansion of space appears to be accelerating for reasons no one fully understands. And the most basic measurements of that expansion, made by different methods, refuse to agree with each other.
Key figure
95%
of the universe is made of dark matter and dark energy, neither of which has been directly detected or fully explained.
The Big Bang and what came before
The Big Bang is not an explosion in space. It is the expansion of space itself, beginning from an extremely hot, dense state roughly 13.8 billion years ago. Within the first fraction of a second, the fundamental forces separated, matter formed, and the seeds of every galaxy were planted as tiny density fluctuations in the primordial plasma.
What happened before that moment remains one of the deepest questions in physics. During the Planck era, the universe's very first instant, both general relativity and quantum mechanics break down. Some physicists propose that our cosmos bounced from a collapsing predecessor inside a black hole. Others argue that the cyclic universe model, in which expansion and contraction repeat forever, cannot be killed despite decades of theoretical attacks.
What is the cosmic microwave background?
The cosmic microwave background (CMB) is the oldest light in the universe, released about 380,000 years after the Big Bang when the plasma cooled enough for photons to travel freely. It fills all of space and carries a faint pattern of temperature variations that maps the structure of the early universe. Nearly every major cosmological discovery of the past 30 years has used the CMB as evidence.
Dark matter and dark energy
Dark matter makes up about 27% of the universe's total energy content. It does not emit, absorb, or reflect light, but its gravitational effects are visible everywhere: in the rotation of galaxies, the bending of light around massive clusters, and the large-scale structure of the cosmic web. Despite decades of experiments, no detector has directly captured a dark matter particle.
Dark energy is even more mysterious. It accounts for roughly 68% of the universe and drives the accelerating expansion of space. But recent observations suggest it may not be constant. Three independent experiments now agree that dark energy appears to be weakening over time, a finding that would overturn the simplest cosmological models. The DESI experiment's measurements of baryon acoustic oscillations add further evidence that something unexpected is happening.
Meanwhile, the expansion itself may not be accelerating at all, according to a controversial analysis by Korean cosmologists. And ancient sound waves from the Big Bang suggest Earth may sit inside a billion-light-year void, potentially explaining why different methods of measuring the expansion rate keep disagreeing.
The structure of the universe
The universe is not uniform. Matter clumps into galaxies, galaxies into clusters, and clusters into vast filaments separated by enormous voids. This cosmic web was seeded by quantum fluctuations in the first moments after the Big Bang and sculpted by gravity over billions of years.
Some structures push the limits of what cosmology predicts should exist. The Big Ring, a 1.3-billion-light-year formation, is one of seven structures large enough to challenge the cosmological principle, the assumption that the universe looks the same in every direction at large scales.
Simulating this complexity requires extraordinary computing power. The Frontier supercomputer ran the first cosmological simulation including gas, stars, and black holes together, testing dark energy and alternative gravity models against observed reality.
Open questions
Cosmology is a field defined by its unsolved problems. Where is the center of the universe? The question itself is wrong: the universe has no center, and every point has equal claim to being the middle of everything. Could our universe exist inside a black hole? The math permits it. Could the universe end without warning through vacuum decay? Physicists ask why it hasn't already.
And the multiverse hypothesis remains the most polarizing idea in the field: either our universe is one of countless others, or the question is not science at all. The debate continues.
Cosmology on Science Reader
We cover cosmology as the evidence evolves. Here are some of our articles:
- Dark Energy Is Weakening. Three Experiments Now Agree.: DESI, the Dark Energy Survey, and the South Pole Telescope all point the same way.
- Baryon Acoustic Oscillations: DESI's Evidence for Changing Dark Energy: Dr. Becky unpacks how DESI used sound waves frozen in the cosmic web.
- The Cosmic Expansion May Not Be Accelerating After All: A controversial finding suggests the standard model of expansion needs rethinking.
- Ancient Sound Waves Reveal Cosmic Void Around Earth: Could our local environment explain cosmology's most stubborn measurement problem?
- Why Physicists Struggle to Kill the Cyclic Universe: The idea that expansion and contraction repeat forever refuses to die.
- Black Hole Universe: Did Our Cosmos Bounce Instead of Bang?: Our universe may have formed from a gravitational bounce inside a massive black hole.
- Inside the Planck Era: When Physics Runs Out of Answers: At the universe's first moment, all known physics breaks down.
- The Strange Math That Places Our Universe Inside a Black Hole: A density coincidence that cosmologists keep revisiting.
- The Ring That Shouldn't Exist: A 1.3-billion-light-year structure that challenges the cosmological principle.
- The Universe Could End Without Warning: Vacuum decay could destroy the cosmos instantly. John Ellis explores why it hasn't.
- Where Is the Center of the Universe?: You're asking the wrong question.
- Multiverse: A Journey Through Parallel Worlds: Do we live in a universe that is just one of many?
- The Silence Between Black Holes: Black holes colliding in silence could help explain cosmic expansion.
- Supercomputer Simulates Universe With Full Physics for First Time: The Frontier supercomputer tests dark energy and gravity models against reality.
From the Science Reader Glossary
- Cosmic Microwave Background | Dark Matter | Dark Matter Detection
- Dark Energy Models | Hubble's Law | Neutron Star Equation of State
FAQ
What is cosmology?
Cosmology is the branch of physics and astronomy that studies the origin, structure, evolution, and ultimate fate of the universe as a whole. It uses observations of distant galaxies, the cosmic microwave background, and gravitational waves to build models of how the universe works at the largest scales.
What is dark energy?
Dark energy is the name for whatever is causing the expansion of the universe to accelerate. It makes up roughly 68% of the universe's total energy content. Despite decades of study, its nature remains unknown. Recent observations suggest it may be changing over time rather than staying constant.
What happened before the Big Bang?
Current physics cannot answer this definitively. Some models propose the Big Bang was a bounce from a previous collapsing universe. Others suggest time itself began with the Big Bang, making 'before' a meaningless concept. Several competing theories exist, but none has been confirmed by observation.
Will the universe end?
Most current models predict the universe will continue expanding forever, with galaxies drifting apart and stars eventually burning out in a 'heat death.' However, if dark energy strengthens, it could tear apart all matter in a 'Big Rip.' And vacuum decay, though unlikely, could end the universe without warning at any moment.
