The Big Bang
The Big Bang is the mysterious origin of the universe about 13 billion years ago from a state of inconceivably great density, called a singularity by physicists. Scientists have no explanation for this event, and it is virtually certain that they never will have an explanation. One reason why is that even though scientists can look back in time by looking farther out in space, no matter how big a telescope we ever build we cannot look farther back in time than a time about 380,000 years after the Big Bang. That is the time when the cosmic background radiation was generated. The cosmic background radiation forbids us from ever being able to see anything that happened during the first 100,000 years of the universe's history. The Big Bang is therefore a mystery locked behind a door that we never can open. This is one mystery that will not be solved in your lifetime.
Dark Energy and the Cosmological Constant
Possibly the second biggest mystery is the mystery of the cosmological constant and dark energy. The cosmological constant is an energy density associated with empty space. Based on the latest Planck results, scientists says that this mysterious dark energy is 68% of the total mass energy of the universe.
Quantum mechanics provides reasons why a vacuum should actually have an energy density, reasons relating to the spontaneous appearance of short-lived virtual particles. The mystery is not there is dark energy, but that the observed amount of dark energy in the universe is more than a million billion trillion quadrillion quintillion sextillion septillion times smaller than the amount of dark energy predicted by quantum field theory, which predicts a vacuum energy density about 10120 times larger than the dark energy we observe. This problem is sometimes called the vacuum catastrophe.
What makes this mystery particularly compelling is that if the dark energy or cosmological constant was even a trillionth as much as that predicted by quantum field theory, life couldn't have evolved in our universe. So you could call this a “we shouldn't be here, but somehow are” mystery.
Another mystery is the enigma of dark matter. 63% of the matter in the universe is believed to be some form of matter that has not yet been discovered. Scientists says this type of matter cannot be seen with our telescopes, and neither emits nor absorbs light. Nobody has ever observed the smallest speck of this type of matter for even the smallest fraction of a second.
For each type of stable particle, scientists say there is an antiparticle. Matter that is made up of antiparticles is called antimatter. When matter and antimatter is combined, all the matter and antimatter is converted to energy (one reason why antimatter is incredibly dangerous stuff). Scientists imagine that the Big Bang should have produced equal amounts of matter and antimatter, but now we live in a universe with abundant matter, and no observable antimatter. What happened to all the antimatter?
This problem, discussed here, is still unsolved. Some physicists have taken some stabs at explanations, but no explanation has been been generally accepted. This problem is another “we shouldn't be here, but somehow are” mystery. If the universe had equal amounts of matter and antimatter at the time of the Big Bang, they presumably would have combined, leaving nothing but energy in the universe.
I discussed this mystery earlier on this blog in my entry Four Insanely Eerie Things About the Electron. The mystery is why it is that protons have exactly the same electric charge as electrons. Each proton is 1836 times more massive than each electron, so it is surprising that each proton has the same charge as each electron, and even more surprising that the proton charge matches the electron charge to nineteen decimal places as mentioned here (a coincidence on which our existence depends).
One might guess that there is just some fundamental reason why subatomic electric charges come in only one amount of 1.60217657 ×10−19 coulomb. But the standard model of physics says that protons are made of three quarks, and that each quark has a smaller electric charge, a charge of exactly one third of 1.60217657 ×10−19 coulomb. So the mystery of charge quantization could be stated in a different way: why does the charge of a quark happen coincidentally to be exactly one third of the charge of an electron, with the match being equal to 19 decimal places? Electrons are not made of quarks, so this is very strange indeed.
The Mystery of Existence
The mystery of existence is simply the mystery of why anything exists at all. In other words, why is there something rather than nothing? The simplest possible state of existence is eternal absolute nonexistence: a state in which nothing exists (no God, no universe, nothing). So why did not this elegantly simple state of existence (the state of absolute nothingness) prevail, rather than our messy, complicated universe? You can't answer the question by imagining (in a quantum mechanical way) that a vacuum is unstable, because an unstable quantum mechanical vacuum is something, rather than nothing, and the question is why something exists rather than nothing.
I have no answer to this age-old question, but I have the suspicion that if you knew the answer to why there is something rather than nothing, then you would have some Rosetta Stone that would give you the answer to a host of other mysteries.
Cosmic Fine Tuning
The term fine-tuning is widely used for cases in which some number in nature has some wildly improbable but favorable value. Nowadays the phrase is actually rather neutral, meaning some natural coincidence that may or may not be due to blind chance. We have numerous and dramatic cases of fine-tuning in the universe, which have been commented on and debated during the past few decades by scientists and non-scientists. A fairly complete description may be found here.
A specific example may be found in this scientific paper, where the authors conclude, “Even with a change of 0.4% in the strength of N-N force, carbon-based life appears to be impossible, since all the stars then would produce either almost solely carbon or oxygen, but could not produce both elements.” More dramatically, here a scientist mentions fine tuning of the cosmological constant to one part in 10120, fine-tuning of the fine-structure constant to one part in 105, fine tuning of the Higgs Boson to 1 part in 1017 (known as “the hierarchy problem”), fine-tuning of cosmic inflation to one part in 1011, and fine tuning “implied by entropy” to as much as 1 part in 1010 multiplied by itself 123 times “according to Penrose” (which is a number so extreme it can't be depicted in the typographical system of this blog).
Chance or design? You be the judge.
The origin of life is the mystery of how life got started billions of years ago on our planet. One of the reasons why the problem is so hard to solve is that all life on Earth is based on DNA, and DNA is a very complicated molecule. We can't simply think that one day there was nothing but a bunch of warm chemicals in a pond, and the next day there arose the first DNA molecules. Presumably there were some intermediate steps or phases that allowed a transition between mere chemicals and DNA molecules, but scientists do not have a clear understanding of what those steps were.
The Origin of Consciousness
The origin of consciousness is the problem of how it is that consciousness could have originated long ago from mere protoplasm. To a person who is a reductionist, and who believes that all human consciousness is merely a chemical and electrical by-product of the brain, this problem may not seem like much of a mystery. But the origin of consciousness has seemed like quite a mystery to those who have considered how different Mind is from Matter. To some, the idea of mind arising from mere matter seems as hard to explain as the inverse (the idea of matter arising from mere mind, as might happen if you thought into existence an apple).
Here is another way to consider the mystery of the origin of consciousness. Let us define a philosophical zombie (for lack of a better term) as a person who acts like a human but has no real inner consciousness or self or private thoughts or inner emotions. We can imagine a race of such beings existing on our planet rather than human beings. Biologically, such a race seems every bit as plausible to exist as the human race (or even more so, since less is required from the brain). So why doesn't such a race exist rather than our race?
Bonus Mystery: The Rectangular Galaxy
The galaxy of LEDA 074886 is not one of the top mysteries of the universe, but I will mention it merely because it makes a nice visual. The mysterious thing about it is its shape: an eerie rectangular shape. How did that happen?
Credit: AWM Graham