It is amazing to consider how many accomplishments humankind has come to achieve with the modernization of technology and science through the years. However, when you begin to ponder the all of the remaining questions that have been left unanswered, you realize that we still have a long way to go with uncovering all of the universe’s mysteries and unexplained phenomenon. Even the basic question of What is the Universe? is given a bleak Wikipedia answer that might leave you a bit disappointed. The fact of the matter is that science is more complicated than you think and much of what you do know about the universe is all just, well… speculation.

We can make speculation much more sophisticated by labeling them as “theories” and interlinking them with other sophisticated theories. However, there is no mathematical or real scientific explanation for even some of the most fundamental questions one might have in life. Even the wisest philosophers have been left with a huge question mark upon studying and debating these major theories. The reality is that there are many mysteries about the universe that physicists and astronomers will never be able to solve. The following fifteen questions are among the top unanswered questions to think about.

15. Can there be life and existence outside of earth?

Many scientists presume that we’re not alone in the universe and that there could very well still be life outside of Earth among some of the other billions of planets. An overwhelming interest in aliens and extraterrestrial existence is nothing new. For years, many have claimed to have seen UFO’s and encountered aliens, although there’s still much skepticism as to whether or not these claims are true or perhaps just a figment of human imagination. And recently, NASA has been discussing the possibility of  life beyond Earth within the near future. People will eventually be able to live on other Earth-like planets and populate them as well.

14. Why is the sun’s outer layer ‘too hot’?

For years, scientists have debated over the question of why the sun’s outer layer is much hotter than its surface. In fact, its outer atmosphere is three hundred times hotter than its surface (which is already 10,340 degrees Fahrenheit). There is some evidence that suggests that this is due to regular explosions or bursts of heat (called nanoflares) from the sun which affect the solar atmosphere (known as the corona) as well. But now, scientists are also debating the presence of a hot plasma that isn’t necessarily caused by nanoflares, which would explain the higher temperature of the outer layer.

13. Will we ever be able to travel through time?

As bizarre as it sounds, some scientists have argued that traveling back in time is theoretically possible. According to a LiveScience interview with professor Charles Liu, astrophysicist at the City University of New York, “In this four-dimensional space-time, you’re only able to move forward in time.” Regardless, scientists are continuing to develop proposals for time travel including an approach that involves the use of wormholes which are essentially tunnels that connect different regions of space-time. Another popular approach is through the use of cosmic strings which are tubes that loop around the entire universe. Both approaches are extremely complicated and impractical projects and therefore, for now, have not been attempted.

12. Why do stars explode?

Stars spend their lives fusing different elements via nuclear reactions into heavier elements, and yet this is something they can only do for so long. Eventually, their core turns to iron and collapses in on itself which causes a powerful shock wave and then an explosion, known as a supernova. The exact details of what goes on inside of a star leading up to a supernova along with what happens within the explosion itself still remain a great scientific mystery. Although scientists are developing more sophisticated explanations and probabilities, it is still impossible to forecast when a star is about to approach the tipping point of a supernova.

11. How did a rectangular galaxy happen?

Most galaxies have three dimensional disc-like shapes. However, in 2012, astronomers spotted a very unusual celestial body that appeared to be a rectangular-shaped dwarf galaxy, also known as the LEDA galaxy. It rest approximately 70 million light years away and was discovered by a ground-based Japanese telescope, the Subaru. Scientists have speculated that this galaxy was perhaps formed as the result of two triangular galaxies colliding, but no one can really explain for sure where the LEDA got its unique, rule-bending shape. Because of the way it appears from earth, scientists can’t even be certain whether or not it really has a spiral disc-like structure.

10. What is gravity?

Newton once provided us with a clean, simple understanding of gravity but that was all turned upside down with Einstein. With as much as we know about gravity and its obvious role in the universe, scientists to this day still struggle with understanding what this force actually is. Also, since it is a force, another puzzling question is why is it so weak in comparison to electromagnetism and nuclear force? And since the other forces in nature have opposites, what is the opposite of gravity and why does it only pull? All of these mysteries, including whether or not we’ll ever have a quantum theory of gravity, have led some scientists to question whether or not gravity is even force.

9. Are there other universes out there?

To date, the Hubble has revealed that there are an estimated 100 billion galaxies or so in the observable universe. And as our telescope technology is rapidly improving, this number will most likely double to about 200 billion galaxies. However, to add to the mystery, theoretical physicists are now speculating that there may be multiple other universes out there as well, living beyond what we can observe. If their speculations are true, then the laws of physics may differ drastically within these other universes. Although it all sounds like a bunch of science-fiction for now, astronomers are actually searching for physical evidence.

8. What’s in like inside the Black Hole?

We all know what it’s like outside of the Black Hole, since that’s where we live, but nothing in existence can tell us what it’s like inside of it. The mystery of the Black Hole starts with its boundary – there’s no real physical barrier. Instead, it has a horizon which divides the two gravitational pulls within and outside of the Black Hole. The reason why we can’t see the inside of a Black Hole, is that an object would have to move faster than the speed of light just to escape it. Therefore, any scope we’d send inside of it would never escape it.

7. How old is the universe?

Regardless of the fact that scientists and philosophers have developed several theories that have attempted to answer this great mystery in an extremely sophisticated fashion, the question is still highly debatable and one that will probably never be solved. The most popular argument held by scientists today is that the earth is 13.772 billion years old, and even this number is an uncertain estimation that is likely off by plus or minus 59 million years. The main limitation that we have with determining the age of the universe is that we also don’t have a firm grasp on the values of the cosmological parameters.

6. How do animals migrate?

We know why animals need to migrate. They migrate throughout the year to escape the changing temperatures and find favorable resources in order to sustain life. What scientists don’t know is how these animals know how to navigate to and from their destinations year after year. One hypothesis is that they use landmarks such as mountains and rivers to help them find their way. Another is that they use the stars, sun, and moon as navigational tools. And the most probable explanation is that they use the Earth’s magnetic field to guide them. Whatever the method, there currently is no way of knowing.

5. Why and how do we get to sleep?

Sleep is one of the all-time greatest mysteries of both science and medicine. First of all, there really is no scientific explanation as to why our bodies are regulated by a circadian rhythm while other organisms sleep at odd hours (owls) or not at all (bull frogs). We spend one-third of our lives sleeping in order for our body to properly function. There are numerous scientific arguments for why sleep is needed and how we can benefit from sleep, including brain plasticity and memory health. However, if sleep is so important to humans, then how is it that no human has ever died from not sleeping? Also, drug companies are still eagerly searching for a cure for insomnia, as how the human body gets to sleep is another great mystery.

4. Where is all the lithium?

In the very beginning of the universe, there was an abundance of hydrogen, helium, and lithium. Today, we still have plenty of hydrogen and helium which make up all of the Universe’s mass. However, there is a great mystery as to what happened to all of the lithium? We only have about a third of the lithium that we should have. Modern scientific studies have suggested that some of this lithium may have been mixed into the core of stars, and would therefore not be observable by any of our telescopes. This is a favorable argument by scientists because hey, if you can’t see it, then that’s probably where it is, right?

3. What is dark matter?

To date, roughly 68% of the universe is made up of this mysterious energy we call “dark energy.” Another 27% of the universe consists of “dark matter.” The remaining 5% is all of the normal matter that we’ve ever observed with our telescopes and instruments, including everything on Earth. Although we have no clue as to what dark matter really is, we are certain about what it is not. It is not any of the following: 1) planets or stars, 2) dark clouds, 3) antimatter, and 4) large black holes. At this point in time, we still have little to no explanation as to what dark matter really is.

2. Did the Big Bang really happen?

The Big Bang is the most popularly accepted explanation by scientists for how the universe was formed. According to Einstein’s theories, our universe formed from a tiny point called a singularity. However, although most scientists accept this theory, no one can form a logical and agreeable explanation as to what came before the singularity, or what the properties the point of singularity even consisted of. The reality is that the Big Bang is a major speculation, and beyond it, we are pretty clueless. Because of our lack of answers to this very primary question, our explanation for numerous other mysteries including the actual age of the universe are all just sophisticated speculations as well.

1. How will the universe end?

This is perhaps the scariest question to think about, should you ponder on it enough. The possible speculations to this question are even more obscure than to how the universe came to being. The most popular two arguments are that 1) it will eventually collapse in on itself (a theory known as the Big Crunch), 2) everything will freeze out (Big Chill), 3) everything will burn up (a theory known as “the heat death of the universe”), and 4) everything including all of the stars and galaxies will tear apart (Big Rip). All of these theories are scary to think about, and none are certain.