Topic Hub
The universe keeps its secrets well. This hub gathers our investigations into the biggest open questions — are we alone, what are those signals, and where might life be hiding — into one guided tour.
The guided journey
From the biggest question to the strangest signal — each stop is a line of inquiry in the search for answers.
Stop 1 of 5
Start with the biggest question of all. The galaxy should be crawling with civilizations — so where is everybody? The paradox that frames everything below.
Read “The Fermi Paradox: Where Is Everybody?”
Cosmic MysteriesThe galaxy should be crawling with civilizations. Seventy-five years after Enrico Fermi asked the question over lunch, the silence is still deafening — and the possible answers keep getting stranger.
Stop 2 of 5
One line of evidence: a distant ocean world where astronomers thought they saw a molecule made only by life — then the signal wobbled. The biosignature debate, honestly told.
Read “A Sharper Spectrum of K2-18 b Reopens the Ocean World Debate”
Cosmic MysteriesFresh transmission data tightens constraints on methane and carbon dioxide in the atmosphere of the sub-Neptune 124 light-years away.
Stop 3 of 5
Closer to home: an ice moon with twice Earth's ocean water, possibly venting it into space. NASA's Clipper mission is on its way to find out.
Read “Are Europa’s Water Plumes Real? Clipper Is Built to Find Out”
Cosmic MysteriesHubble glimpsed geysers on Jupiter’s icy moon, then lost them. Europa Clipper’s instruments are now calibrated to catch one in the act.
Stop 4 of 5
The technosignature hunt. If advanced civilizations exist, their engineering might outshine their biology — and we may have already spotted candidates.
Read “Dyson Spheres: Have We Already Spotted an Alien Megastructure?”
Cosmic MysteriesIn 1960, Freeman Dyson described how to find giant alien civilizations: look for the waste heat of star-sized machines. Sixty-six years later, astronomers are still looking — and the hunt just got interesting.
Stop 5 of 5
End with the pure mystery: a signal from deep space that repeats every 16 days like clockwork, and nobody knows what makes it.
Read “Fast Radio Burst Repeats Every 16 Days, Stumping Astronomers”
Cosmic MysteriesFRB 20180916B keeps firing on schedule from a spiral galaxy 500 million light-years away. New data rules out the simplest explanations.
The overview
Every age of astronomy has been defined by its mysteries. The planets' wandering paths became orbital mechanics; the "spiral nebulae" became other galaxies. Today's mysteries are bigger: we have mapped the cosmic microwave background to exquisite precision and still cannot say what 95% of the universe is made of — or whether anyone else is out there wondering the same thing.
The search for life runs on two tracks. Biosignatures are chemical hints of biology: gases like oxygen, methane, or dimethyl sulfide in an exoplanet's atmosphere that are hard to make without life. The K2-18 b saga shows both the promise and the peril — a tantalizing signal, then the painstaking work of checking it. Technosignatures look for engineering instead: Dyson spheres capturing starlight, industrial pollutants, or deliberate beacons. A sufficiently advanced civilization might be far easier to spot by its works than by its microbes.
Meanwhile, some mysteries may have nothing to do with life at all. Fast radio bursts release staggering energy in milliseconds; a few repeat on clockwork schedules no known physics cleanly explains. They are a reminder that the universe still produces phenomena we have no framework for — the modern equivalent of the pulsars that were briefly, half-seriously, labeled "LGM" for Little Green Men.
And the most promising address for life may be close to home. Europa, Enceladus, and a growing census of ocean worlds suggest liquid water — the one ingredient life as we know it cannot do without — is common. If the plumes on Europa are real, we may get to taste an alien ocean within a decade, no drilling required.
The features below follow the investigation the way scientists do: start with the biggest question, chase each line of evidence, and sit honestly with what remains unexplained. Begin at stop one.
Quick answers
The contradiction between the high probability of extraterrestrial civilizations — billions of stars, billions of Earth-like planets, billions of years — and the complete absence of evidence for them. Proposed solutions range from "we are first" to "they are hiding" to "interstellar travel is effectively impossible." Our feature walks through the strongest answers.
A hypothetical megastructure built around a star to capture its energy — the ultimate technosignature of an advanced civilization. Physicist Freeman Dyson proposed it in 1960. No confirmed Dyson sphere exists, but infrared surveys have flagged a handful of candidates, including stars from the Hephaistos project, that deserve a closer look.
Millisecond-long flashes of radio energy from deep space, some releasing as much energy as 500 million Suns. Most never repeat, but a few — like FRB 20180916B — pulse on precise schedules. Magnetars (hyper-magnetic neutron stars) explain some; the clockwork repeaters remain unexplained.
We do not know. In 2025, astronomers reported a possible biosignature gas (dimethyl sulfide) in its atmosphere, then follow-up analysis weakened the signal. K2-18 b remains one of the most promising "hycean" ocean-world candidates — and a case study in how science corrects itself in public.
Europa's subsurface ocean holds about twice the water of all Earth's oceans, kept liquid by tidal heating from Jupiter. If the plumes spotted by Hubble are real, they offer a way to sample that ocean without drilling. NASA's Europa Clipper, en route now, is designed to settle it.
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