When the Black Death ran rampant through cities in the Middle Ages, no one knew exactly how or why the awful disease spread. After many generations, we figured out that rat fleas and bacteria were to blame. It was a watershed moment for the power of science.

Centuries later, the brightest minds continue to investigate difficult, bewildering scientific questions every day. Yet even with artificial intelligence and brilliant minds connecting with more computing power than our species has ever known, we still don't have all of the answers. In fact, some people might argue that we're just now learning to ask the truly big questions.

What happens to us after we die? How did so much life appear on our planet when others seem devoid of any species at all? Who, if anyone, pulls the strings of our universe? Is it some all-powerful god in control or are there physical and mathematical principles driving the engine of our existence?

Sometimes, after centuries of missteps, we humans finally stumble into real answers to real questions, such as why diseases spread. Other times, we're left grasping into the darkness of our own ignorance and wondering what any of it really means. But humanity will keep trying to eliminate questions and give us the true answers.

10. Why Do We Dream?

Typically, you dream during the rapid-eye movement portion of the sleep cycle, but researchers don't know why you dream in the first place. Andresr/Getty Images

During your latest shut-eye adventures, you chopped the head off of a six-legged rabbit while wearing a neon pink bonnet and screaming, "Gesundheit" at the top of your lungs. You aren't sure whether that dream means something unless it's that perhaps you consumed too many hallucinogens during college or merely ate some bad carrots yesterday.

Scientists and sleep experts know when people normally dream. Typically, this happens during the rapid-eye movement(REM) portion of the sleep cycle. You can see when a person (or even your cat or dog) is experiencing REM sleep because their eyes zip to and fro and their bodies may twitch and jerk, too. The brain's electrical patterns are very active in this phase, just like when you're awake.

But researchers don't really know why you dream. It may be a way of reflecting on or releasing the stress of everyday life, or even an unconscious way of helping you unravel challenging experiences. It could be a way that your mind protects itself from threats and dangers.

It could be a biochemical way for your brain to sort, file or store short- or long-term information. Perhaps dreams are a way to reconcile your past and present experiences to prepare and steel you for the future.

Regardless of their purpose, dreams are a cornerstone of the human experience. They entertain and haunt us and serve as reminders that our inner world is just as deep and strange as the exterior world all around us.

9. How Can We Eliminate Cancer?

Scientists doubt we'll ever have a cure for every type of cancer, but perspective and treatment for cancers is evolving. Mark Kostich/Getty Images

Cancer is one of the deadliest diseases. While modern medicine can cure cancer, treatment isn't effective for all kinds. Each year, more than half a million people die from various cancers in the United States alone. Its familiarity, however, does not make it any less frightening.

Cancer takes many forms and affects many parts of the body, but the hallmark of these diseases is uncontrollable cell replication. Tumors expand and spread, ruining bodies and causing death.

The growth happens due to DNA damage. DNA, of course, provides instructions for all bodily functions, including cell growth. That damage may happen because of certain lifestyle factors, such as sun damage, tobacco smoking or prolonged exposure to carcinogenic chemicals.

By some estimates, we could prevent more than a third of cancers by avoiding cancer-causing habits. However, life choices are only part of the equation. Other factors play a role, too. Human genome issues may make some people more predisposed to develop certain types of cancer, even if they live totally healthy lives.

The myriad variables and unique genetic makeup of humans make some scientists doubt that we'll ever have a cure for every type of cancer. There are just too many environmental assaults and minute bodily malfunctions for any one magic bullet to attack.

The good news is that our perspective and treatment for cancers is evolving. Each year, we understand new aspects of the disease. Therapies keep improving, lessening suffering and adding quality of life. So although we may never fully defeat cancer, we'll keep beating it back, making our lives better and diagnoses less terrifying.

8. What Happens When You Die?

The subject of the afterlife — or lack thereof — is one of the oldest quandaries troubling humankind. Muchlis Akbar/EyeEm/Getty Images

Just as much as we like to understand how life began, we also want a better idea of what happens to us after we die. And hey, there are billions of people who already know the answer to that scientific question. Unfortunately, they can't tell us about it because, well, they're all dead.

The subject of the afterlife — or lack thereof — is one of the oldest questions troubling humankind. Will we all float off into eternal bliss? Will the evil among us be damned to the pits of hell? Will our consciousness merely vanish once our bodies have expired? Or will we all be reincarnated as bizarre fish or fluffy cats?

Scientists do understand the beginning stages of death. They know how the human body begins shutting down. Like store employees turning off the lights at a megastore after closing hours, your body's cells begin to blink off, one by one, until your heart and brain cease activity.

What happens after your brain clicks off, though, is still a complete mystery. Many people who have gone through near-death experiences and then come back to life speak about tunnels of light or flashbacks to life occurrences or conversations with loved ones who've passed away.

All of those experiences could have biological origins, perhaps spurred by a lack of oxygen or wild biochemical fluctuations.

Of the many questions we face about our existence, this is one that we may never, ever get an answer to. Instead, we'll all be left yearning, searching for some sort of meaning in the death.

7. Are We Alone in the Universe?

There could be as many as 40 billion habitable planets in our galaxy alone. Estate of Stephen Laurence Strathdee/Getty Images

While the study of cosmology and quantum physics has given us some answers on the universe, we still have no idea if we're the only intelligent lifeforms in our solar system. If that's the case, the universe is unimaginably lonely.

Other researchers say there's almost no way that Earth is the only headquarters for life — there could be as many as 40 billion habitable planets just in our Milky Way galaxy, so imagine other universes. That's an awful lot of potential for alien life.

There are some necessary requirements for life to arise. Not only does a planet need the right mix of elements (like all the carbon) and conditions, but there also has to be a spark that gives rise to living creatures. Then, of course, those creatures have to somehow evolve into beings with intelligence.

Even to modern human science, the simplest of our planet's lifeforms are still an extremely complex stew of chemical reactions and cells. We don't really understand how they emerge, evolve and survive in an incredibly diverse range of environmental conditions. That makes finding, identifying and communicating with alien beings much more complicated.

In spite of those challenges, researchers at NASA think we may find traces of life in next couple of decades. More powerful telescopes could be one key to finding it.

Or it could be that life here is just a statistical aberration, an accident of the weirdest kind. Maybe this odd swamp of a planet really is a jewel of the universe, unduplicated and unlike any place else, anywhere.

Yet we know that water and similar gases and elements exist on many other planets. If we keep looking and happen to find even a shred of evidence, such as fossilized remains or tiny bacteria, it seems more likely that somewhere across the stars another species is also looking to the heavens and pondering potential neighbors somewhere in the universe, too.

6. Where Does Consciousness Come From?

Brain scans display how active our brains really are, flickering with constant activity as our 100 billion nerve cells ceaselessly fire. Du Cane Medical Imagery/Getty Images

We humans have an awareness of our surroundings and also of ourselves. Our minds teem with internal dialogues and questions about who we are and our purpose in the world. We are, as far as we know, the only creatures with this sort of active consciousness. We also have no idea where this consciousness comes from.

Our brains, of course, are the central computers of our bodies, controlling biological functions and helping us think our way through all of life's loops and hurdles. Brain scans display how incredibly active our brains really are, flickering with constant activity as our 100 billion nerve cells ceaselessly fire, like a compact yet enormously complex digital network.

But the brain is not the mind. Electrical activity doesn't explain how a physical substance can create a nonphysical condition like consciousness. Some religions explain consciousness as a gift from God, embedded into our bodies to guide us through this world.

Scientists veer more toward biological origins — they see consciousness as a collection of biological processes that build toward more complicated thinking that eventually culminates in self-awareness.

Scientists have determined that animals, such as dogs, almost certainly have consciousness, but that it's a lower (or different) level of awareness than that of humans.

5. How Many Species Are on Earth?>

By some estimates, we’ve still only accounted for 1.5 million species, or around 15 percent of the total number on Earth. Danita Delimont/Getty Images

Earth is home to a dazzling array of creatures and plants. Pink flamingos fill the skies, humongous elephants stomp through the savannas, and weird fruits and ferns hide in crevasses everywhere. We'll never know how many different species roam our planet. There are just too many. But that doesn't stop scientists from trying to determine that elusive number.

Botanist Carl Linnaeus realized two and half centuries ago that humans needed a system for keeping track of our planet's species. He began classifying both plants and animals using taxonomic language that named, ranked and classified creatures and plants.

After generations of work, by some estimates, we've still only accounted for 1.5 million species, or around 15 percent of the total number. That means the majority of organisms still need adequate description. That's especially true for undervalued and underappreciated species such as fungi, of which we've really only described maybe 10 percent.

In contrast, we've done a pretty good job with our fellow mammals, most of which we have already logged.

All of the numbers are simply statistical guesses, so we may never really know if they're accurate. Perhaps the bigger concern is that species seem to be disappearing at a rate faster than at any time since dinosaurs vanished 65 million years ago. After all, if creatures are disappearing en masse, we humans could be next.

4. Is Reality Real?

Perhaps the things and people in our lives are merely illusions, much like "The Matrix." Archive Photos/Getty Images

Human reality is a slippery concept. Anyone who wakes up from a vivid nightmare knows what it's like to feel stuck somewhere between a memory and a dream. Those experiences lend credence to the idea that there are limitations to our senses. Perhaps our eyes, ears and sense of smell don't really tell the whole story about the reality all around us.

Perhaps the things and people in our lives are merely illusions. How do we know that any of these objects and creatures actually exist? Maybe they're constructs of our own internal mechanisms, spawned by our subconscious for unknown purposes. The universe could be a hologram, a "Matrix" style computer creation meant to entrap our minds and enslave us to a nefarious purpose.

Scientists and physicists aren't sure we'll ever understand the nature of reality. The deeper we delve into physics, the stranger the mechanics of our universe become.

We keep discovering new particles and fundamental forces, from molecules to atoms, that drive our bodies and our world. It's entirely possible that the universe could be made up of dozens or thousands of dimensions that we'll never experience in any direct way.

It's unlikely that the scientific rabbit hole we've burrowed will ever end. No matter how smart our collective species become, reality will always be an abstraction that we can never quite pin down.

3. How Did Life Begin?

A 2.72 billion-year-old bacterial deposit near the Nullagine River in Pilbara region of Western Australia. Auscape/Getty Images

Our planet is teeming with trees, grasses, birds and bees. It's also swarming with countless bacteria. All of it is life, and all of it reproduces to keep its kind alive.

But how in the world did life begin in the first place? How did a mass of cells shift from an inert collection of organic molecules into a wiggling and sometimes even intelligent being?

The short answer is: We don't know exactly how life originated. There's the off chance that 4 billion years ago, aliens dropped off a few microbes and let them run wild. And of course, many religions have supernatural explanations for life's origins.

Many scientists think that life is a natural progression for planets that feature the necessary ingredients for biology, such as carbon, hydrogen, oxygen and other fundamental building blocks. With the right spark — say, a bolt of lightning — those cornerstones slowly develop into the cell walls and DNA that are suited to reproducible life. Researchers are continually running experiments like this in laboratories, hoping to zoom in on the formula for making life.

Try as they might, it's a mystery how those bits of nonliving parts assembled themselves into actual living creatures. It could be that we are as yet still ignorant of the characteristics that truly define life. Or maybe we're blind to the principles of physics that really make life tick. No matter the case, the search for life's origins will undoubtedly continue for a long while.

2. Is Time Travel Possible?

Ken Kapalowski in his replica of the DeLorean time machine from "Back To The Future." Getty Images

As science-fiction concepts go, time travel is one of the most captivating. It's hard not to wonder what it would be like to zip back into history to witness a Roman battle in action, or to breathe air that hadn't been touched by constantly burning fossil fuels. It's perhaps even more intriguing to ponder what our world would look like if you could instantly peek 1,000 years into the future.

As it turns out, time travel might not be fiction. It may be that we just haven't quite figured out how to make it work for us.

One possibility is wormholes, which are bridges of a sort that could help people move through time and space. If you could breach an opening into a wormhole, you could theoretically enter it and then wind up on the other side of the galaxy in a different place and time.

We could try traveling at the speed of light, at which point your world slows down greatly compared to the one you leave behind. Our current science says that nothing can move as fast as light, though, and even if we could, it might tear our bodies apart.

Maybe we could orbit massive black holes, which have such incredible gravitation pull that they actually slow down time. Hang out by a black hole and your experience of time would be roughly halved compared to life on Earth. If you returned 10 years later by your perception, your family would have aged 20 years in that time.

Or perhaps we could use cosmic strings, the so-called cracks in the universe, to navigate time. These strings (which are also sometimes loops) have so much mass that they may actually cause the space-time around them to fluctuate.

Manipulating any of these scenarios might grant us the power to finally realize time travel. Even if we can figure out the science, though, there are numerous paradoxes that might make time travel unfeasible or downright dangerous. So for now, traveling in time is still simply the stuff of books and movies.

1. Is the Universe Truly Infinite?

Science can’t really tell us yet whether the universe is infinite or finite, but researchers do have theories. Claudio Ventrella/Thinkstock

When you look up on a dark night and see the uncountable stars scattered throughout the skies, it's easy to think of the universe as infinite. Or maybe you see those illuminations as similar to the glow-in-the-dark stars on your bedrooms ceiling, just pretty decorations to behold, a high but dead-end ceiling. Either way, science can't really tell us yet whether the universe is infinite or finite.

As with everything else, researchers do have theories.

After analyzing maps generated by observations made with the Baryon Oscillation Spectrographic Survey (BOSS), a super-powerful telescope in New Mexico, one group determined that the universe has an exceedingly flat floor plan. The survey was based on observations of "only" 1.2 million galaxies, which is a drop in the universal bucket, but it's a strong clue that our universe isn't sphere-shaped.

So is that flatness infinite? It's impossible to say. One reigning thought is that the Big Bang is causing the universe to constantly expand faster than the speed of light. Because we can't see beyond the speed of light, we'll never really know if there's an edge to the universe.

We'll likely have to spend our lives wondering about the true nature of the universe's size. It's a mystery, like so many others, that's overflowing with a captivating allure that draws our minds but never offers any real answers.

Whether you're struggling with how big the universe might be or how life originated on Earth, our existence is brimming with bizarre twists of nature that we can't explain and may never understand. Those questions may trouble us and hound us for our entire lives — but they are also an essential part of the human experience.

Unanswered Questions FAQ

What is an unanswerable question?

An unanswerable question is one with no known answer. It is a mystery that continues to stump.

What is the difference between an unanswered question and a mystery?

It is possible for unanswered questions to have answers that simply haven't been stumbled upon yet. A mystery, however, is something that remains unresolved after attempting to resolve it.

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Author's Note: 10 Questions That Science Can't Answer Yet

It's easy for modern people to look back on their ancestors with more than a little disdain. Living in caves, really? Where's your dignity, ape men? But most of us understand that 100 years from now, our grandchildren will look back at us with the same kind of mild sympathy, thinking of us as primitive and unknowing. The evolution of human science and knowledge has that kind of effect. So maybe in 20 or 50 years, this list of unanswered questions will seem quaint and naïve. It's more likely, though, that at least some these questions are going to stand the test of time.

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