—— In recent centuries, we have solved many mysteries and overcome many problems using science, but some questions leave even the brightest minds scratching their heads. Five mysteries which human intelligence has failed to unravel to this very day.


When astrophysicist Stephen Hawking was asked some time ago whether he saw himself in a tradition with Albert Einstein, Galileo Galilei, and Max Planck, he gave a modest answer: “All scientists are trying to build on the pyramid of human knowledge. I hope I have been able to add a small pebble,” he said, at the same time describing the paradox that even the greatest achievements usually mean little progress in the big picture of science.

As you may recall, Albert Einstein developed the theory of relativity, revolutionizing human understanding of space and time. Galileo Galilei used experiments to prove that all objects in a vacuum fall to the ground at the same speed, founding the science of kinematics. Max Planck is considered the founder of quantum physics, which forms the basis of the technologies that modern conveniences such as smart phones and televisions use. Stephen Hawking himself became famous with his work on black holes.

All four researchers made crucial contributions to the advancement of science, and all four possessed exceptionally high intelligence. However, even after their incredible findings, countless questions of how our world works remained unsolved. “We don’t know even one millionth of one percent of things,” is how Thomas Edison, the inventor of the light bulb, summed up this sobering realization. The knowledge of how little we know is at the core of science but often continues to provide for all the more amazement in the face of cutting-edge technologies. We present five puzzles that remain unsolved to this day and explain why they are so difficult to figure out.


They swallow up stars, absorb light, and condense matter in their innermost core into an infinitely large mass that ignores all known scientific laws. What sounds like some unreal place in a science fiction movie is actually a description of one of the most fascinating puzzles in physics. Since their discovery, black holes have captivated scientists – and still leave them asking fundamental questions today.

They have now reached a consensus that black holes exist and are formed when stars have used up their energy and collapse due to gravity. The extreme gravitational force causes an infinite amount of mass to be concentrated into a very small space. Everything that is sucked into black holes disappears in them irrevocably, making it impossible to observe their interior.

“Black holes are the darkest secret of our galaxy,” was the statement of the Nobel Committee when it announced the 2020 laureates. Because black holes defy direct observation, scientists are forced to derive their knowledge about them from theories and indirect evidence from experimental physics.

This is a laborious process that nevertheless has generated a kind of basic consensus over decades. Most researchers today assume that in the middle of the Milky Way there is a black hole that determines the orbit of the stars at the center of our galaxy. The approximate mass of this black hole is believed to be about four million suns, and it is located about 26,000 light years from Earth. To this day, researchers cannot see the black hole directly, but they can make conjectures about it by observing what is happening around it.


On average, humans spend about 20 years of their lives asleep. It is important for regeneration, helps to process cognitive stimuli, and saves energy. There are many more reasons why we need to sleep, but science still does not know what the primary reason behind it is.

One thing is certain: Our body regenerates during sleep, breaking down free radicals that can damage our genetic material and building up proteins that have a positive effect on health. However, there is no particular reason that the body would have to switch off consciousness to do these things. Some psychologists therefore suspect that the human brain needs a deep sleep phase to do “sorting” work. Memories stored in the hippocampus while awake could be transferred to our cerebrum during deep sleep, and the waking state would interfere with this processing. Another theory claims that sleep is needed not to store memories but to erase them. Without this deletion work, the argument goes, the brain would quickly become overloaded.

Existing theories are not necessarily mutually exclusive, of course. While we sleep, many processes run in parallel, all of which have important benefits. Since sleep has only been studied more intensively since the middle of the 20th century, it has not yet been possible to connect all the dots.


Forward/backward, up/down, left/right: That is all the spatial dimensions we are familiar with. It is all the more difficult for us to imagine that there could be 5, 15, or even 30 more, but that is exactly what many physicists presume. Something that is so hard to imagine could be the key to explaining other unsolved mysteries of particle physics and cosmology and possibly explain why three-dimensional space has proven to be particularly resilient. But to move forward in this area, it is first necessary to find the previously undiscovered dimensions.

Albert Einstein was the first to add a fourth axis to the three-dimensional X-Y-Z coordinate system. Extension by the factor of time was revolutionary in physics, allowing scientists to converge on a mathematically correct representation of reality. However, physics has still not arrived at a precise understanding of this fourth dimension. This has brought the field to a standstill – at least in terms of experimental detectability. Theoretically, many scientists are devoting thought to many more new dimensions.

String theory, for example, takes 10 existing dimensions as its basis. This theory was then extended in M-theory, which conjectures 11 dimensions, and in boson string theory, which assumes there are 26. Common to all theories is the attempt to understand the hitherto known forces of nature in a uniform way, to describe them, and to gain further knowledge along the way. The precise number of dimensions is therefore unimportant to many scientists. Rather, they are interested in the information that each individual dimension may contain. Theoretical physicist Lee Smolin describes the unsolved puzzle in this way: “If string theory or loop quantum gravity were the solution itself, we would know it by now. They may be clues, small parts of the answer, they may contain important insights, but they they really offer us no more than that.”


To determine the fastest route, just open a map, compare the possible routes, and choose the shortest one? You might think that, but the basic problem of garbage truck routing belongs to the so-called millennium problems of mathematics, with prize money awaiting the person who can solve it, but so far no satisfactory answer has been arrived at.

The garbage collection puzzle is also known as the “messenger problem” or the “traveling salesman problem” and has kept mathematicians busy for years. Some have come up with approaches for calculating the shortest distance, but the necessary computing power far exceeds the polynomial time, which is seen as a value for the practical solvability or unsolvability of a problem. A problem is referred to as solvable in polynomial time if it can be solved by an algorithm whose required computation time grows no more than polynomially as the size of the input to the problem increases. Here is an example: To add five numbers, we need to use five calculation steps. The runtime is therefore just as long as the input time. If these values no longer match, it is no longer polynomial time.

Currently, no one has an idea of how the garbage collection problem could be solved. While it is true that an algorithm exists that can quickly check a proposed solution for correctness, an algorithm that could also quickly solve the problem in reverse has yet to be found. Whether such an algorithm could ever be developed is now considered doubtful.


As a rule, people only believe in what they can see, which is why the universe is so difficult to comprehend. We can see the sun, moon and stars as well as gas and dust, but current findings show that they account for only about five percent of the total matter in the universe. What does the rest consist of? Science currently believes that there is an invisible mass – dark matter.

By its very nature, the biggest problem in exploring this invisible mass is its invisibility. Nevertheless, various astronomical phenomena have led researchers to assume that it exists. Without dark matter, rotating galaxies would immediately be flung apart by centrifugal force, for example. So far, the only real evidence of dark matter is its gravitational pull. Researchers estimate that it makes up about 27 percent of our universe.

As if that were not complicated enough, another invisible mass can be added to the puzzle: dark energy. It counteracts the gravitational pull of dark matter and could be responsible for the universe expanding forever. Although dark energy and dark matter are such polar extremes, they have one thing in common: there are promising approaches to learning more about them, but astronomy is still in the dark when it comes to determining exactly what they are.