Quick answer: The biggest unsolved mysteries of the universe include what dark matter and dark energy are (together about 95% of the universe), what happened before or at the Big Bang, why there is more matter than antimatter, why measurements of the universe’s expansion rate disagree (the Hubble tension), what happens inside black holes and whether we are alone. Scientists can measure many of these effects precisely but still can’t explain their causes.
We know the universe is about 13.8 billion years old, that it is expanding, and that it contains roughly two trillion galaxies in the observable part alone. Yet the more precisely astronomers measure the cosmos, the clearer it becomes how much is still unexplained. Here are 12 of the biggest open questions, what we know about each, and what remains a mystery.
Unsolved mysteries of the universe at a glance
| Mystery | What we know | What we don’t know |
|---|---|---|
| Dark matter | About 85% of all matter; detected by its gravity | What particle, if any, it is made of |
| Dark energy | About 68% of the universe; speeds up expansion | What it is, and whether it changes over time |
| The Big Bang | The universe was hot and dense 13.8 billion years ago | What caused it, and if “before” means anything |
| Matter vs antimatter | The Big Bang should have made equal amounts | Why matter won |
| Hubble tension | Two methods give different expansion rates | Whether it’s an error or new physics |
| Black hole interiors | Black holes exist and have been imaged | What happens at the center |
| Alien life | Billions of planets lie in habitable zones | Whether life exists anywhere else |
1. What is dark matter?
Galaxies spin too fast, and light bends too much, for the visible matter in them. An invisible form of matter, about five times more abundant than ordinary matter, seems to provide the extra gravity. Decades of experiments have not detected a dark matter particle. Read the full guide: what is dark matter?
2. What is dark energy?
In 1998, two teams studying distant exploding stars found that the expansion of the universe is speeding up, a discovery that won the 2011 Nobel Prize in Physics. The cause was named dark energy. Recent results from the Dark Energy Spectroscopic Instrument (DESI) have hinted that dark energy may be weakening over time, which, if confirmed, would change our picture of the universe’s future.
3. What happened at the very beginning?
The Big Bang theory describes how the universe expanded and cooled from a hot, dense state. Most cosmologists also accept cosmic inflation, a burst of extremely rapid expansion in the first tiny fraction of a second, which explains why the universe looks so smooth and flat. But what drove inflation, and what, if anything, came before it, are open questions.
4. Why is there more matter than antimatter?
Every particle has an antimatter twin, and when they meet they destroy each other in a flash of energy. The Big Bang should have produced equal amounts of each, leaving nothing behind. Instead, a tiny excess of matter, around one extra particle per billion pairs, survived to form everything we see. Physicists have found small differences between matter and antimatter, but not enough to explain this.
5. The Hubble tension: how fast is the universe expanding?
Measuring the expansion rate from the early universe’s afterglow gives about 67 km/s per megaparsec. Measuring it from nearby stars and supernovae gives about 73. The gap has survived years of checks, including with the James Webb Space Telescope. It may be a hidden measurement error, or a sign that something is missing from our cosmic model.
6. What happens inside a black hole?
General relativity predicts a point of infinite density at the center, but quantum physics suggests that can’t be the whole story. Stephen Hawking’s prediction that black holes slowly evaporate also created the information paradox: does information about what fell in get destroyed? (See the black hole information paradox explained.) See what is a black hole?
7. How did giant black holes grow so fast?
The James Webb Space Telescope has found supermassive black holes millions of times the Sun’s mass less than a billion years after the Big Bang. Growing that big that quickly is hard to explain, so they may have started from unusually large “seeds”.
8. Are we alone in the universe?
Astronomers have confirmed thousands of exoplanets, and many stars have planets in their habitable zones. Yet we have found no clear sign of life elsewhere. This puzzle, why the universe seems so quiet, is called the Fermi paradox. See the search for alien life.
9. Is the universe infinite?
The observable universe is about 93 billion light-years across, limited by how far light has traveled since the Big Bang. Measurements show space is very close to geometrically “flat”, which is consistent with an infinite universe but does not prove it.
10. Is there a multiverse?
Some versions of inflation and string theory suggest our universe could be one of many. It is an interesting idea, but there is currently no way to test it directly, so many scientists consider it speculation.
11. What causes fast radio bursts?
Fast radio bursts are millisecond flashes of radio waves from distant galaxies, first detected in 2007. In 2020, one was traced to a highly magnetic dead star (a magnetar) in our own galaxy, partly solving the mystery. But not all bursts seem to have the same cause.
12. Is there a hidden planet in our solar system?
Even our own backyard has mysteries. The strange orbits of some distant icy objects may point to an unseen planet several times Earth’s mass. Read is there a Planet Nine?
Which mysteries might be solved soon?
- Planet Nine: the Vera C. Rubin Observatory’s sky survey should find it or rule out much of where it could be.
- Dark energy: DESI, ESA’s Euclid mission and NASA’s Nancy Grace Roman Space Telescope will measure cosmic expansion more precisely.
- Early black holes: Webb continues to study the first galaxies.
- Life on other worlds: missions to Europa and studies of exoplanet atmospheres are searching for signs of life.
Frequently asked questions
What is the biggest mystery of the universe?
Many scientists would choose dark energy or dark matter, because together they make up about 95% of the universe and we don’t know what either is.
How old is the universe?
About 13.8 billion years, based on measurements of the cosmic microwave background.
What is outside the universe?
No one knows, and the question may not have a meaningful answer. The universe is not thought to be expanding into anything; space itself is stretching.
Will the universe end?
If dark energy stays constant, the universe will keep expanding and slowly go cold and dark over unimaginably long times. If dark energy changes, other endings are possible.
Can we ever see the edge of the universe?
We can see back to the cosmic microwave background, light released about 380,000 years after the Big Bang. That is the edge of the observable universe, not necessarily the edge of the universe itself.
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- What is a black hole?
- What is dark matter?
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Sources
- Planck Collaboration (2020). “Planck 2018 results. VI. Cosmological parameters.” Astronomy & Astrophysics. doi:10.1051/0004-6361/201833910
- Riess, A. G. et al. (1998). “Observational evidence from supernovae for an accelerating universe and a cosmological constant.” The Astronomical Journal. doi:10.1086/300499
- DESI Collaboration (2025). Results from Data Release 2 baryon acoustic oscillation measurements.
- Riess, A. G. et al. (2022). “A comprehensive measurement of the local value of the Hubble constant.” The Astrophysical Journal Letters.
- CHIME/FRB Collaboration (2020). “A bright millisecond-duration radio burst from a Galactic magnetar.” Nature.
- NASA Science: “Universe”. science.nasa.gov