A catalog of what nobody knows.

• Nº 001

  Why do we sleep?

  Every animal with a nervous system does it. It is dangerous, costly, and non-negotiable, and nobody knows what it is buying.

  Neuroscience · First posed Antiquity · Open

• Nº 002

  What is dark matter?

  Five independent lines of evidence say most of the matter in the universe is invisible. Fifty years of searching have not caught a single particle of it.

  Cosmology · First posed 1933 · Open

• Nº 003

  Why does anesthesia work?

  We have been switching human consciousness off on demand since 1846. We still cannot say what the switch is.

  Medicine · First posed 1846 · Open

• Nº 004

  How did life begin?

  Somewhere on the early Earth, chemistry became biology. Every laboratory attempt to repeat the trick has stalled at the same gap.

  Origins · First posed Antiquity · Open

• Nº 005

  Why is ice slippery?

  A question a child can ask, studied since Faraday, still argued in physics journals. The textbook answer you learned is mostly wrong.

  Physics · First posed 1850 · Open

• Nº 006

  Does P equal NP?

  If checking an answer is easy, is finding one easy too? A million-dollar question about the nature of difficulty itself.

  Mathematics · First posed 1971 · Open

• Nº 007

  How do birds sense magnetic fields?

  A robin can feel a force you cannot. The leading explanation requires quantum mechanics operating inside a warm, wet eye.

  Biology · First posed 1966 · Open

• Nº 008

  Why do we dream?

  Every night you hallucinate vividly for about two hours, then forget most of it. No one can tell you what that is for, or whether it is for anything.

  Neuroscience · First posed Antiquity · Open

• Nº 009

  What is dark energy?

  About two thirds of the universe is something that pushes space apart. The leading explanation misses by a factor with 120 zeros.

  Cosmology · First posed 1998 · Open

• Nº 010

  Why do we age?

  Nothing in physics says a body must wear out. Some animals barely do. Ours does, on schedule, and the reason is still argued about.

  Biology · First posed Antiquity · Open

• Nº 011

  What is ball lightning?

  Thousands of people have watched a glowing sphere drift through a room during a storm. Physics still cannot say what they saw.

  Physics · First posed 1638 · Open

• Nº 012

  Is the Riemann hypothesis true?

  One sentence from 1859 would, if proved, bring order to the prime numbers. It has been checked trillions of times and proved zero times.

  Mathematics · First posed 1859 · Open

• Nº 013

  Why is the Sun's corona hotter than its surface?

  Step back from a fire and the air gets cooler. Step off the Sun's surface into its atmosphere and the temperature rises three hundredfold.

  Astrophysics · First posed 1939 · Open

• Nº 014

  What causes Alzheimer's disease?

  We can see the plaques, name the proteins, and now even slow the decline a little. The actual cause is still an open verdict.

  Medicine · First posed 1906 · Open

• Nº 015

  How does turbulence work?

  We have had the exact equations for chaotic fluid flow since the 1840s. We still cannot solve them, and we cannot even prove they behave.

  Physics · First posed 1883 · Open

• Nº 016

  Where is everybody?

  Billions of old, sunlike stars. Planets around most of them. Six decades of listening. One universe, apparently silent.

  Astronomy · First posed 1950 · Open

• Nº 017

  How did language begin?

  The defining human ability left no fossils. The question was once so hopeless that the field formally banned it.

  Linguistics · First posed Antiquity · Open

• Nº 018

  Why do we hiccup?

  A reflex with its own dedicated neural circuitry, present before birth, apparently good for nothing. It has a job or it is a souvenir. Nobody knows which.

  Biology · First posed Antiquity · Open

• Nº 019

  What is consciousness, physically?

  Three pounds of matter produces an inner life. Every fact in this catalog was experienced by someone. How that happens is unknown at the deepest level we can ask.

  Neuroscience · First posed Antiquity · Open

• Nº 020

  How does the placebo effect work?

  A sugar pill, honestly labeled as nothing, can still relieve pain. The body runs a pharmacy that expectation can unlock, and we hold only a partial inventory.

  Medicine · First posed 1955 · Open

• Nº 021

  Where did Earth's water come from?

  The planet formed close to a young star, hot and dry by every rule we know. Yet here are the oceans. The delivery route is still being argued.

  Planetary science · First posed 1951 · Open

• Nº 022

  Why is there more matter than antimatter?

  The Big Bang should have made matter and antimatter in equal amounts, which then annihilate to light. Something cheated, one part in a billion. You are made of the discrepancy.

  Physics · First posed 1928 · Open

• Nº 023

  Does every number fall to one?

  Halve it if even, triple it and add one if odd, repeat. A child can play the game. Nobody alive can prove it always ends.

  Mathematics · First posed 1937 · Open

• Nº 024

  Why do zebras have stripes?

  Darwin and Wallace argued about it. A century and a half later the leading answer involves flies, and the mystery has moved rather than died.

  Biology · First posed 1871 · Open

• Nº 025

  How do memories outlive their molecules?

  The proteins storing your oldest memory were recycled thousands of times since it formed. Something persists. What, exactly, is the question.

  Neuroscience · First posed 1894 · Open

• Nº 026

  What triggered the Cambrian explosion?

  Life spent three billion years as slime, then produced most animal body plans in a geological moment. Darwin called it a valid objection to his theory. The trigger is still not settled.

  Origins · First posed 1859 · Open

• Nº 027

  What are fast radio bursts?

  Millisecond flashes of radio from other galaxies, each briefly outshining its host. Thousands are catalogued. The engine is still not identified.

  Astronomy · First posed 2007 · Open

• Nº 028

  Why does time flow one way?

  The laws of physics run fine in reverse. Eggs still refuse to unscramble. The one-wayness of time is real, measurable, and not yet explained at the bottom.

  Physics · First posed 1876 · Open

• Nº 029

  How do high-temperature superconductors work?

  Certain ceramics conduct electricity with zero loss at temperatures far too warm for the standard theory to allow. Four decades on, the mechanism is still contested.

  Physics · First posed 1986 · Open

• Nº 030

  Why do cats purr?

  The most familiar sound in domestic life, produced by a mechanism only recently glimpsed, for reasons that remain genuinely unclear.

  Biology · First posed Antiquity · Open

• Nº 031

  Why do humans cry emotional tears?

  Every animal with eyes lubricates them. Only humans weep at funerals, weddings, and films. The function of the most human fluid is unestablished.

  Biology · First posed 1872 · Open

• Nº 032

  Why is yawning contagious?

  You may yawn while reading this entry. Nobody can tell you why, on either count: what yawns are for, or why they spread.

  Biology · First posed Antiquity · Open

• Nº 033

  What is the glass transition?

  Cool a liquid past crystallization without letting it crystallize and it quietly becomes solid without ever changing structure. Physics cannot say what kind of event that is.

  Physics · First posed 1948 · Open

• Nº 034

  What happens to information that falls into a black hole?

  Quantum mechanics forbids information from being destroyed. Black holes looked like they destroy it. The verdict has shifted; the mechanism is still missing.

  Physics · First posed 1976 · Open

• Nº 035

  How many species share the planet?

  Around 2.2 million species have names. The true total is unknown to within an order of magnitude, and possibly several.

  Biology · First posed 1758 · Open

• Nº 036

  How does sound make a bubble glow?

  Squeeze a bubble in water with sound waves and it emits flashes of light, from a spot briefly hotter than the surface of the Sun. The lamp works; the manual is unfinished.

  Physics · First posed 1934 · Open

• Nº 037

  How fast is the universe expanding?

  Two impeccable measurements of the universe's expansion rate disagree, and the disagreement has survived every attempt to blame the instruments.

  Cosmology · First posed 2016 · Open

• Nº 038

  What causes migraines?

  A billion people carry a brain that periodically attacks them, complete with light shows and a warning phase. The full circuit has never been traced.

  Medicine · First posed Antiquity · Open

• Nº 039

  Why can salamanders regrow limbs and we cannot?

  An axolotl rebuilds a severed leg, perfectly, bones and nerves and all. Mammals seal the wound and call it done. Both use nearly the same genes.

  Biology · First posed 1768 · Open

• Nº 040

  What causes déjà vu?

  For a few seconds, the present moment feels like a memory, and you know it isn't. The glitch that watches itself has no confirmed mechanism.

  Neuroscience · First posed 1876 · Open

• Nº 041

  How do rogue waves form?

  Sailors' tales of walls of water were dismissed for centuries, until a drilling platform measured one. The ocean makes monsters; the recipe is disputed.

  Physics · First posed 1995 · Open

• Nº 042

  Where do the highest-energy particles come from?

  Single atomic nuclei arrive from space carrying the energy of a fast-pitched baseball. Nothing we know of should be able to throw them.

  Astrophysics · First posed 1962 · Open

• Nº 043

  Why do we blush?

  An involuntary broadcast of exactly the feeling you most want to hide, wired into one species only. Darwin called it the most human expression, and he could not explain it.

  Biology · First posed 1872 · Open

• Nº 044

  Where does the proton's spin come from?

  The proton spins. Its three quarks, added up, provide barely a third of the total. Most of the spin of ordinary matter is bookkept somewhere still unmeasured.

  Physics · First posed 1987 · Open

• Nº 045

  What is ME/CFS?

  An illness that follows infection, worsens with exertion, and disables millions has no biomarker, no approved treatment, and no accepted mechanism. The patients were right; the science is late.

  Medicine · First posed 1956 · Open

• Nº 046

  What is whale song for?

  Humpback males sing structured, evolving songs that sweep across oceans like hit records. Half a century of listening has not settled what the songs accomplish.

  Biology · First posed 1971 · Open

• Nº 047

  What killed the ice-age giants?

  Mammoths, giant sloths, sabertooth cats: the large animals of every continent except Africa vanished in a geological instant. The suspects are climate and us.

  Origins · First posed 1796 · Open

• Nº 048

  Why do we have fingerprints?

  The ridges on your fingertips are famous for identifying you. That is a use humans invented. What they are actually for is still being tested.

  Biology · First posed 1892 · Open

• Nº 049

  Does hot water freeze faster than cold?

  Aristotle said yes. A Tanzanian schoolboy insisted yes. Careful labs say maybe not. This entry is unusual: the open question is whether the effect exists at all.

  Physics · First posed Antiquity · Open

• Nº 050

  How does gravity fit with quantum mechanics?

  Physics runs on two magnificent theories that contradict each other about what reality is. Every attempt to marry them remains unconfirmed by a single experiment.

  Physics · First posed 1916 · Open

• Nº 051

  Where do European eels spawn?

  No one has ever seen a European eel egg, or a European eel spawning, in the wild. Aristotle guessed. We are still guessing better.

  Biology · First posed Antiquity · Open

• Nº 052

  How does lightning get started?

  The electric fields measured inside thunderclouds are roughly ten times too weak to spark. Lightning strikes anyway, about a hundred times a second, worldwide.

  Atmospheric science · First posed 1925 · Open

• Nº 053

  Why are the Moon's two faces so different?

  The side of the Moon we see is dark lava plains; the side we never see is pale, cratered, and thicker-skinned. Sixty years of spacecraft have not settled why.

  Planetary science · First posed 1959 · Open

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