pThree years ago, Vox launched Unexplainable, a podcast about unanswered questions and what we learn when we explore the unknown. There’s a line I think about all the time from our very first episode.

“Whatever we know is provisional,” Priya Natarajan, a Yale physicist, told us about research on dark matter. But the sentiment also applies to science overall. “It is apt to change. What motivates people like me to continue doing science is the fact that it keeps opening up more and more questions. Nothing is ultimately resolved.”

Unexplainable isn’t about how scientists don’t know anything. Science is a process of narrowing a gap between the questions we have and the capabilities of our tools and know-how to answer them. In many cases, that gap appears closed. No one doubts, for instance, the existence of gravity.

But even then, it is a scientist’s job to have intellectual humility, or at least to be open to the idea that there’s still a piece missing — as there is with gravity — knowing the results could just end up confirming what they thought in the first place.

Really, science is about a big question: How do we know when we’ve completely learned something?

What this series has taught us is that answering the question is a journey. Sometimes the stories on that journey are exciting — like what happens when NASA launches a staggeringly powerful observatory into space. Sometimes they are frustrating, especially when answers to a question are held back by powerful forces like scientific funding, perverse incentives, or stigma.

Most often, though, the stories are deeply human: We ask questions because we’re trying to understand our imperfect bodies, our beautiful but fragile world, and our place in the universe just a bit better.

We’re drawn to questions because they are optimistic. They invite us to dream of a better world in which they are answered, where the gaps between questions and our capabilities to answer them are smaller. Scientific knowledge is a gift we can give the future. It’s worth getting right.

Here are some of the questions that astounded us the most.

1) What is the universe made out of?

NASA

If you go outside on a dark night, in the darkest places on Earth, you can see as many as 9,000 stars. They present as tiny points of light, but in reality, they are massive infernos. And while these stars seem astonishingly numerous to our eyes, they represent just the tiniest fraction of all the stars in our galaxy, let alone the universe.

All the stars in all the galaxies in all the universe barely even begin to account for all the stuff out there. Most of the matter in the universe is unseeable, untouchable, and, to this day, undiscovered.

Scientists call this unexplained stuff “dark matter,” and they believe there’s five times more of it in the universe than normal matter — the stuff that makes up you and me, stars, planets, black holes, and everything we can see in the night sky or touch here on Earth. It’s strange even calling all that “normal” matter because, in the grand scheme of the cosmos, normal matter is the rare stuff. But to this day, no one knows what dark matter is.

So, how might scientists actually “discover” it?

Further reading: Dark matter holds our universe together. No one knows what it is.

2) How did life start on Earth?

DeAgostini/Getty Images

For decades, scientists have been trying to re-create in labs the conditions of early Earth. The thinking is, perhaps if they can mimic those conditions, they will eventually be able to create something similar to the first simple cells that formed here billions of years ago. From there, they could piece together a story about how life started on Earth.

This line of research has demonstrated some stunning successes. In the 1950s, scientists Harold Urey and Stanley Miller showed that it’s possible to synthesize the amino acid glycine — i.e., one of life’s most basic building blocks — by mixing gases believed to have filled the atmosphere billions of years ago and adding heat and simulated lightning.

Since then, scientists have been able to make lipid blobs that look a lot like cell membranes. They’ve gotten RNA molecules to form, which are like simplified DNA. But getting all these components of life to form in a lab and assemble into a simple cell — that hasn’t happened.

So what’s standing in the way? What would it mean if scientists succeeded in creating life in a bottle? They could uncover not just the story of the origin of life on Earth, but come to a shocking conclusion about how common life must be in the universe.

Further reading: 3 unexplainable mysteries of life on Earth

3) How did dogs evolve from wolves?

Christian Charisius/picture alliance via Getty Images

Wolves and dogs are nearly genetically identical, sharing 99.9 percent of their DNA (and are more similar to each other than we are to our close animal relatives, like chimps), yet they behave differently. Wolves “still have all of their natural hunting behaviors which dogs don’t have,” Kathryn Lord, a scientist who studies the evolution of behavior, says. “In the wolves, everything you greatly fear seeing in a dog pup is totally normal.”

Scientists still don’t know what precisely caused wolves and dogs to diverge from one another some 20,000 years ago. There are two main hypotheses. Either we humans domesticated wolves through a painstaking and dangerous process (possibly involving breastfeeding wolf pups!), or the wolves, essentially, domesticated themselves by venturing closer and closer to our trash (i.e., food).

The answer is more than just trivia. “A better understanding of how this might have happened long ago might give us a better understanding also to how animals and plants and such today might be able to — or not able to — adapt to us,” Lord says.

And to find out, Lord has been playing with some puppies:

Further reading: How gray wolves divided America

4) Can animals feel grief?

Olivier Morin/AFP via Getty Images

In 2018, a mother orca carried the carcass of her dead calf for 17 days, covering thousands of miles of ocean. The journey inspired many media reports, but also, one big question: Was this mother orca grieving?

Similar stories have popped up across the animal kingdom: of a dog refusing to leave its deceased owner’s grave, of elephants apparently convening in “mourning,” of geese that appear to grieve the loss of a mate and refuse to eat.

Though it’s easy to look at these behaviors and assume these animals experience a human-like version of grief, the science of studying animal emotion and death behaviors is much trickier. Some scientists suggest it’s not possible to know the interior life of an animal. Others say there’s a lot to be learned about the evolutionary history of grief if we go with the assumption that this is grief.

“There’s a principle in science of parsimony that was to say if something evolved in one species, it’s very unlikely that, you know, it didn’t also evolve in other species,” says Jessica Pierce, a bioethicist.

On Unexplainable, Pierce and two other researchers help us think through this thorny question: What can we learn from animal reactions to death?

Further reading: Breakups really suck, even if you’re a fish

5) What will animals look like in the future?

Amanda Northrop/Vox

It’s impossible to completely predict how evolution will play out in the future, but that doesn’t mean we can’t try. Reporter Mandy Nguyen asked biologists and other experts to weigh in: What would animals look like a million years from now?

The experts took the question seriously. “I do think it’s a really useful and important exercise,” Liz Alter, professor of evolutionary biology at California State University Monterey Bay told Nguyen. In thinking about the forces that will shape the future of life on Earth, we need to think about how humans are changing environments right now.

Further reading: The animals that may exist in a million years, imagined by biologists

6) What’s the secret to a great romantic relationship?

Getty Images/Westend61

Scientists grapple with the same relationship questions matchmakers, romance authors, poets, and anyone who has ever been single do.

“The big mystery is — do you really know who you want?” says Dan Conroy-Beam, a University of California Santa Barbara psychologist who studies relationship formation. Single people often have an imagined perfect partner, but is this person really the one who will make them happy?

The question seems simple, but it’s not trivial. A lot of time, energy, and heartache goes into finding solid relationships. “In a lot of senses, who you choose as a partner is the most important decision you’ll ever make,” Conroy-Beam says. “That’s going to affect your happiness, your health, and your overall well-being.”

Scientists don’t have all the answers, and they often disagree on which answers are even possible. But I found that their hypotheses — along with some advice from matchmakers and relationship coaches — can help us think through how love starts and how to maintain it once it’s found.

Further reading: What science still can’t explain about love

7) Where the heck does our moon come from?

HUM Images/Universal Images Group

Before the moon landings, scientists thought they knew how the moon came to be, assuming it formed a lot like other planets did: Debris and dust leftover from the formation of the sun essentially clumped together to form rocky worlds like Earth and the moon.

But then, Apollo astronauts brought samples back from the lunar surface, and those rocks told a totally different story.

“Geologists had found that the moon was covered in a special kind of rock called anorthosite,” Unexplainable producer Meradith Hoddinott explains on the show. “Glittery, bright, and reflective, this is the rock that makes the moon shine white in the night sky. And at the time, it was thought, this rock can only be formed in a very specific way: magma.”

The indication there was magma means the moon must have formed in some sort of epic cataclysm: “Something that poured so much energy into the moon that it literally melted,” Hoddinott says. Scientists aren’t precisely sure how it all played out, but each scenario is a cinematic story of fiery apocalyptic proportions.

Further reading: How Apollo moon rocks reveal the epic history of the cosmo

8) How does sound become hearing?

Getty Images/iStockphoto

Sound enters our ears, light enters our eyes, chemicals splash up in our nose and mouth, and mechanical forces graze our skin. It’s up to our brains to make sense of what it all means and create a seamless conscious experience of the world.

In the 1970s, psychologist Diana Deutsch discovered an audio illusion that made her feel like her brain was a little bit broken. “It seemed to me that I’d entered another universe or I’d gone crazy or something … the world had just turned upside down!” Deutsch recalls on Unexplainable.

Like the visual illusions that trick our eyes into seeing impossible things, the audio illusion Deutsch discovered in the 1970s fooled her ears. Sometimes illusions make us feel like, as Deutsch says, something is off with our minds. But really, these misperceptions show how our brains work.

Illusions teach us that our reality isn’t a direct real-time feed coming from our ears, eyes, skin, and the rest of our bodies. Instead, what we experience is our brain’s best guess.

But how do our brains do this? And how can scientists use that information to help people, invent new tools, or understand ourselves better?

Further reading: What science still doesn’t know about the five senses

9) Why don’t doctors know more about endometriosis?

Sudipta Das/NurPhoto via Getty Images

In people with endometriosis, a disease in which tissue similar to what grows inside the uterus grows elsewhere in the body. It’s a chronic condition that can be debilitatingly painful. Yet doctors don’t fully understand what causes it, and treatment options are limited.

Worse, many people with endometriosis find that doctors can be dismissive of their concerns. It can take years to get an accurate diagnosis, and research into the condition has been poorly funded.

Vox reporter Byrd Pinkerton highlighted how frustrating it can be to suffer from an often-ignored, chronic condition. “It’s just so, so, so soul-crushing to just live in this body day in and day out,” one patient told Pinkerton.

Further reading: Menstrual fluid’s underexplored medical treasures

10) Is there anything alive in the human poop left on the moon?

NASA

During the Apollo moon missions, astronauts went to the moon and, to save weight for returning to Earth, they dumped their waste behind. Across all the Apollo missions, astronauts left 96 bags of human waste on the moon, and they pose a fascinating astrobiological question.

Human waste — and in particular, feces — is teeming with microbial life. With the Apollo moon landings, we took microbial life on Earth to the most extreme environment it has ever been in. Which means the waste on the moon represents a natural, though unintended, experiment.

The question the experiment could answer: How resilient is life in the face of the brutal environment of the moon? And for that matter, if microbes can survive on the moon, can they survive interplanetary or interstellar travel? If they can survive, then maybe it’s possible that life can spread from planet to planet, riding on the backs of asteroids or other such space debris.

Further reading: Apollo astronauts left their poop on the moon. We gotta go back for that shit.

11) Was there an advanced civilization on Earth before humans?

Science Photo Libra/Getty Images

Many scientists have long wondered: Is there intelligent life out in the deep reaches of space? Climate scientist Gavin Schmidt and astrophysicist Adam Frank have a different question: Was there intelligent life in the deep reaches of Earth’s history? Could we find evidence of an advanced non-human civilization that lived perhaps hundreds of millions of years ago, buried in the Earth’s crust?

This is not strictly a “solar system” mystery, but it is cosmic in scope. At the heart of it, Schmidt and Frank are asking: How likely is an intelligent life form on any planet — here or in the deepest reaches of space — to leave a mark, a sign that they existed? And for that matter: Hundreds of millions of years from now, will some alien explorers landing on Earth be able to find traces of humans if we’re long, long gone?

Further reading: The Silurian hypothesis: Would it be possible to detect an industrial civilization in the geological record?

12) What is the definition of “life”?

Mark Ralston/AFP via Getty Images

We know life when we see it. Flying birds are clearly alive, as are microscopic creatures like tardigrades that scurry around in a single drop of water.

But do we, humans, know what life fundamentally is? No.

“No one has been able to define life, and some people will tell you it’s not possible to,” says New York Times columnist and science reporter Carl Zimmer. It’s not for a lack of trying. “There are hundreds, hundreds of definitions of life that scientists themselves have published in the scientific literature,” he says.

The problem is, for every definition of life, there’s a creature or perplexing life-like entity that just sends us right back to the drawing board.

Further reading: What is life? Scientists still can’t agree.

13) How should we define death?

Getty Images

Death used to be fairly self-evident. Someone stopped breathing, their heart stopped beating — they were dead. But new technologies have forced us to ask: When is someone actually dead?

Now, new research is raising a further question: Might it even be possible, in some instances or for just a brief moment, to reverse death? It sounds outlandish, but researchers at Yale University describe how they were able to partially revive disembodied pigs’ brains several hours after the pigs’ death.

If this technology progresses, could it redefine death?

Further reading: There’s a surprisingly rich debate about how to define death

14) What did dinosaurs sound like?

Tayfun Coskun/Anadolu Agency via Getty Images

What would it be like to be near a dinosaur? From fossil evidence, scientists can get a decent sense of what these ancient creatures looked like. But they still don’t know what they would have sounded like. Whereas hard tissues like bone can fossilize and leave us information about dinosaur stature and shape millions of years later, soft tissues — like the muscle and cartilage that help generate sound — do not fossilize as readily.

Many Hollywood depictions of dinosaur roars are not based in scientific reality (the T-Rex roar in Jurassic Park is partially based on an elephant. A mammal! Dinosaurs were reptiles!). So where do scientists start in trying to imagine realistic dinosaur noises? They look to dinosaurs’ closest relatives alive on Earth today.

Further reading: What did dinosaurs actually sound like? Take a listen.

15) Is there such a thing as perfect internet encryption?

Getty Images

Today’s internet is built on a series of locks and keys that protect your private information as it travels through cyberspace. “Encryption is basically like this cloak that wraps your private information,” Unexplainable’s Meradith Hoddinott says on the show. If someone intercepts your message as it travels around the web, “it just looks like random static”

But there’s a fear: With increases in computing power, it’s possible that one day all these locks can be broken.

So cryptographers are trying to probe deep, complicated mathematical theory. They want to know: Could a perfect, unbreakable “lock” even exist?

Further reading: Inside the quest for unbreakable encryption at MIT Tech Review

16) Is it safe to use weed during pregnancy?

Getty Images/fStop

There is really good research out there that shows that if a parent drinks too much alcohol during pregnancy, it can have clear consequences for the child, affecting everything from their weight and size to their cognitive abilities, vision, and hearing. There is also good evidence that smoking cigarettes can harm a fetus.

As Vox reporter Keren Landman found in recent reporting, by contrast, the consequences of cannabis use are less obvious. The studies that have been done have had mixed results. Researchers aren’t entirely clear on whether cannabis use affects birth weights, and while there are some connections drawn between cannabis use in pregnancy and attention, hyperactivity, and aggression in kids, these results are also not clear-cut.

In spite of these mixed results, Landman found that cannabis use in pregnancy is still heavily penalized in states across the US — even in states where the drug is legal. Pregnant parents sometimes use cannabis to help them cope with morning sickness or other pregnancy symptoms, but in many states, they can have their children taken away by child protective services, or even be arrested and jailed.

Why is there such a mismatch between the science and the policy? And how can we improve both, and make parents feel safe discussing cannabis use with their providers?

Further reading: Is weed safe in pregnancy?

17) How will everything end?

NASA, ESA, CSA, STScI, Webb ERO

In the early 1900s, Henrietta Leavitt, a Massachusetts-born “computer” who worked at the Harvard College Observatory, published a discovery that may sound small but is one of the most important in the history of astronomy: She found a way to measure the distance to certain stars.

Over time, scientists kept building on Leavitt’s ruler to measure the universe. As they used these measuring tools, their understanding of the universe evolved. They realized it was far bigger than previously thought, there are billions of galaxies, and it’s expanding: Those galaxies are moving farther and farther away from one another.

Astronomers also realized that the universe had a beginning. If galaxies are moving away from one another now, it means they were closer together in the past — which led scientists to the idea of the Big Bang.

It also led them to realize that the universe may, eventually, end.

Further reading: How scientists discovered the universe is really freaking huge

There are more than 100 episodes of the Unexplainable podcast. Find the complete archive here.