The 95 percent problem
Measurements from the Planck satellite, galaxy surveys and supernova studies agree on a striking budget. Ordinary matter, everything made of atoms, accounts for about 5 percent of the universe's energy content. Dark matter makes up roughly 27 percent. Dark energy, the mysterious driver of accelerating expansion, accounts for about 68 percent.
Those numbers are confirmed in the sense that many independent methods converge on them. What the dark components actually are is entirely open. That combination, a precise measurement of something nobody understands, is what makes cosmology so compelling right now.
Known versus unknown
- In 1933, Fritz Zwicky found that galaxies in the Coma Cluster move too fast to be held by visible mass.
- In the 1970s, Vera Rubin and Kent Ford measured flat rotation curves: outer stars in spiral galaxies orbit as fast as inner ones.
- The Bullet Cluster, studied with the Chandra X-ray Observatory in 2006, shows mass separated from hot gas, strong evidence that dark matter is real stuff, not just a tweak to gravity.
- In 1998, the Supernova Cosmology Project and the High-Z Supernova Search Team found that distant supernovae are fainter than expected: expansion is speeding up.
- No experiment has directly detected a dark matter particle, despite decades of searches deep underground and at the Large Hadron Collider at CERN.
- Whether dark energy is a constant property of space (Einstein's cosmological constant) or something that evolves over time. Recent DESI survey results hint at evolution, but are not yet conclusive.
- Whether a modified theory of gravity could explain some observations instead.
Open files in the log
The Hubble tension
Measurements from the early universe (Planck) give an expansion rate near 67 km/s per megaparsec. Measurements from nearby Cepheids and supernovae, led by Adam Riess's SH0ES team, give about 73. JWST has checked the nearby data and the gap persists. Either there are hidden errors, or new physics is involved.
Fast radio bursts
First reported by Duncan Lorimer in 2007, these millisecond flashes come from far beyond our galaxy. In 2020, one was traced to a magnetar inside the Milky Way, a strong clue. Whether all bursts share that origin is not settled.
'Oumuamua
Spotted by the Pan-STARRS telescope in Hawaii in October 2017, it was the first known interstellar object. It was elongated, showed no visible tail and accelerated slightly. Natural explanations exist, but none is universally accepted.
The Wow! signal
On 15 August 1977, Ohio State's Big Ear radio telescope recorded a strong narrowband signal lasting 72 seconds. Jerry Ehman circled it and wrote "Wow!" It has never been detected again.
The Fermi paradox
Planets are common and the galaxy is old. So where is everyone? The question, attributed to Enrico Fermi in 1950, has dozens of proposed answers and no evidence that settles it.
Matter over antimatter
The Big Bang should have made equal amounts of matter and antimatter, which would have annihilated. Something tipped the balance slightly in favour of matter. Nobody knows what.
How to read a mystery responsibly
Headlines love the word "mystery", and some claims stretch well beyond the data. A useful rule at this observatory: an unexplained observation is not evidence for any particular explanation. The Wow! signal is unexplained; that does not make it alien. Dark matter is undetected; that does not mean gravity is wrong.
Real progress usually comes slowly: better instruments, larger samples, independent teams checking each other. ESA's Euclid telescope, launched in July 2023, is mapping dark matter's influence across billions of galaxies. The Vera C. Rubin Observatory will catch transient events by the million. Some of these files will close in the coming decade. Others may stay open for generations.
Questions of your own? Try the Stargazer FAQ or send a transmission.