Everything else on this site is the part that has been worked out. This page is the other part. These are real questions with no answer yet — not riddles, not mysteries, just places where the honest thing to say is "we do not know", and where somebody alive now may well end up being the one who finds out. They are in order: the first is nearly within reach, the last has had the best people in the world stuck on it for a decade.
The Moon is supposed to be the most boring object in the sky: a dead rock with no air and no weather, which has not done anything interesting in three billion years. And yet, for centuries, careful observers have reported brief flashes, glows and hazy patches on it that were gone within minutes.
Some of these are now completely solved. Meteorites hit the Moon constantly, and with no air to stop them they arrive at full speed and flash on impact. Telescopes watching for exactly this have recorded hundreds of impact flashes, and there is no argument about them at all.
An impact flash lasts a fraction of a second. Some of the reports describe a reddish glow, or a patch going hazy as though something were blurring the view, over tens of minutes. Nothing hitting the Moon can do that. The leading idea is gas escaping from inside — the Moon is not quite as dead as it looks — possibly lifting dust as it goes. It has never been caught cleanly enough to settle.
The honest picture is messy in a way that is worth seeing. A good fraction of historical reports are almost certainly mistakes: bad air, a dirty eyepiece, an eager observer. Of the rest, most are impacts. And there is a residue that nobody has explained and nobody can dismiss. Sorting a real signal out of a pile of unreliable reports is a large part of what science actually consists of, and it rarely looks tidy.
Every kind of particle has an opposite twin, identical but with the opposite charge. Put one of each together and they destroy each other completely, leaving nothing but light.
As far as anyone can tell, the hot early universe described on how the universe began should have made these in equal numbers. Equal numbers would have annihilated down to nothing. There would be light, spreading and cooling forever, and not one atom anywhere.
Something tipped the balance, very slightly. For roughly every billion pairs that destroyed each other, about one particle of ordinary matter was left over. Everything that exists — every star in the catalogue, the Earth, you — is built from that leftover billionth.
The laws of physics are genuinely not quite even-handed between matter and antimatter. That lopsidedness was found in laboratories in 1964 and has been measured many times since. So the idea is not invented to fill a hole — the effect is real.
The lopsidedness that has been measured is far too small. Nowhere near a factor of two out — out by many powers of ten. Something else must have happened in the first fraction of a second, and there is no shortage of proposals and no evidence for any of them.
Here is the most uncomfortable fact in astronomy, and this site is a good place to feel it. Every object in the catalogue — every planet, every star, every galaxy, every scrap of gas — is made of ordinary matter. And ordinary matter is about a twentieth of what is out there.
Both names are honest about being placeholders. "Dark" here does not mean black or hidden — it means we have measured something and cannot say what it is.
Galaxies spin far too fast. Measure how quickly the outer parts of a spiral go round and compare it with the pull of everything you can see in it, and the visible matter is not nearly enough — the outskirts should have been flung off long ago. Something with mass is holding them together, spread out in a vast invisible halo. Our own galaxy has one, and this site lists it as the Milky Way's dark matter halo, because leaving it out would be dishonest about where the mass is.
It is not merely too faint to see. It does not interact with light at all — it casts no shadow and gives off nothing. Experiments buried deep underground have been waiting for decades for a single particle of it to bump into something. So far: nothing.
This one is younger and stranger. In 1998, two teams measuring distances with exploding white dwarfs — the Type Ia supernovae that rung 6 depends on — found the far ones were fainter than they should be. The expansion of the universe is not slowing down under its own gravity, as everyone expected. It is speeding up.
Nobody knows why. "Dark energy" is the label for whatever is doing it, and the label has been in place for a quarter of a century without anyone filling it in.
Thousands of planets are now known around other stars, and some of them are roughly Earth-sized and at roughly the right distance from their star for liquid water — TRAPPIST-1e and Proxima Centauri b are two in this site's own catalogue. Thirty years ago we knew of none.
So: is there life on any of them? The answer is that nobody knows, and it is worth being precise about what kind of not-knowing this is.
It sounds overwhelming and it proves nothing. To turn a big number of planets into a prediction about life you have to multiply it by the chance that life starts on a suitable planet — and nobody has the faintest idea what that number is. It could be one in ten. It could be one in a trillion trillion. Multiply an enormous number by an unknown one and the answer is still unknown.
The reason nobody can pin it down is that we have exactly one example of life to study, and it is us. One example is not a sample you can do statistics on — which is the point of the experiment at the bottom of this page.
What would actually settle it is evidence, and there are two kinds worth watching for. The first is a planet's air: life on Earth keeps our atmosphere in a state it could not hold on its own, and the James Webb Space Telescope can now read the air of some planets around other stars by watching starlight shine through it. The second is something built — a signal, a structure, anything nature would not make.
Every claimed detection so far has either evaporated on closer inspection or is still argued over. That is not a failure. It is what "we do not know" looks like from the inside.
This is the sharpest open problem on the page, because it is not a gap in what we have measured — it is two careful measurements that disagree and will not stop disagreeing.
One way is the distance ladder this whole site is built on: work out how far away galaxies are, rung by rung, see how fast they are receding, and divide. The other way starts from the microwave background described on how the universe began and works forwards using the standard model of cosmology. Both are done by excellent people. They do not agree.
The bars are how uncertain each measurement is. Twenty years ago they were wide enough to overlap and everyone assumed the problem would go away as they narrowed. Both narrowed. The gap stayed exactly where it was.
That leaves two possibilities and no third one. Either somebody has made a subtle mistake that has survived decades of checking by people actively hunting for it — or the standard model of the universe is missing something, and we have been given a genuine clue about what.
Nobody sensible is confident which. That is a good measure of how open a question is: not that people argue loudly, but that the careful ones will not bet.
None of these will be solved by thinking harder. They need better measurements, and the machines to make them are being built or are already working.
| What | What it does | Which question it bites on |
|---|---|---|
| Vera C. Rubin Observatory | Photographs the entire southern sky every few nights for ten years, and compares each picture with the last. | Millions of supernovae for the expansion rate — and anything that flashes, changes or moves, which is how you catch things nobody predicted. |
| JWST | Reads the air of planets around other stars, and re-measures the distance ladder's rungs in infrared. | Life elsewhere; and an independent check on whether the ladder half of the Hubble tension is right. |
| The Extremely Large Telescope | A single mirror 39 metres across, on a mountain in Chile. | Enough light to study small rocky planets directly rather than by inference. |
| LISA | Three spacecraft in formation millions of kilometres apart, measuring ripples in space far longer than any ground detector can. | Distances worked out from the ripples alone, with no ladder at all — a completely independent way to settle the expansion rate. |
That last one is not hypothetical. GW170817 — the two neutron stars whose collision was felt before it was seen — already gave a rough expansion rate from its ripples alone, needing none of the seven rungs. It was not precise enough to decide anything. A few dozen more like it would be.
This one is not about the sky. It is about the trap underneath question 4, and you need an opaque bag and a pile of counters, beads, buttons or coloured paper scraps in two colours.
We have drawn one planet with life on it. That is the entire sample. Anyone who tells you life must be common, or must be rare, is guessing at the contents of the bag from a single counter.
And there is a twist that makes it worse. Imagine the rule was that you were only allowed to reach into the bag if your first counter came out red. Then a red first counter tells you nothing whatsoever about the bag — it was guaranteed by the rule. That is our situation exactly: we could only ever have found ourselves on a planet with life, because we had to be alive to look. Our one example was never going to come out any other way.
It would be easy to read this page as a list of things too deep to ever understand. It is not. Every question here is a question about something measurable, and every one of them has people building equipment to answer it right now.
It is worth remembering how recent the rest of this site is. That the Milky Way is not the whole universe was settled in 1925. What powers the stars was worked out in the 1930s. The first planet around another Sun-like star was found in 1995, and ripples in space were first detected in 2015 — a century after they were predicted, by people who mostly assumed it could never be done.
Every one of those was on a page like this one, once.