We can't see it and we can't feel it, but we can test for it, and nobody knows what it is. In spite of this, scientists think that dark energy makes up around 70% of the universe. It was imagined to explain why galaxies don't just drift apart but instead accelerate away from each other. You can think of it as a repulsive gravity that pushes matter apart. How it works, however, is still a mystery.
The other "dark" substance in our universe. Dark matter, like dark energy, cannot be seen or felt. This elusive substance has some differences to dark energy though; the only way that we have observed it is indirectly. We know that there must be more matter in the universe than we can see because we can measure its gravitational effects, but no one knows exactly what makes up this mysterious stuff.
It's a wave... it's a particle!:
Rays of light have a split personality. They create interference patterns that are typical of waves. They reflect off surfaces, suggesting that they could be a wave or a particle, or both at the same time. They can also be used to liberate electrons from their shells: something that indicates that they are particles. But how does light determine whether it acts as a particle or a wave?
Time, the onward march:
We only get older, not younger. Trees only get taller; they don't return to acorns. Our Sun only ever uses up its fuel, never returning to a cool ball of hydrogen gas. Time only goes in one direction... but why is it impossible for us to reverse the clocks?
We are living in a hologram:
This one boggles the mind. The universe, everything we see and feel and experience, may actually have two spatial dimensions. Think of a 2D hologram, like the one on the back of a credit card: it can have all of the information of a 3D image but in only two dimensions. Some scientists have postulated that our universe is like the hologram on your credit cards: space seems like it has three dimensions, but it may turn out that all we are seeing is a projection from a 2D universe outside of our perception.
Matter and antimatter
There is a definite discrepancy between the ratios of these two substances. There was supposed to be an equal amount of ordinary matter and antimatter – particles with the same mass but opposite charge – in the early universe, but now the universe is overwhelmed with regular matter. Many theories have been thrown around, for example that particle genesis favored one way of creating matter, but nothing conclusive has popped up. The mystery of how matter "won" over antimatter may be revealed in the newly-upgraded Large Hadron Collider at CERN.
The lifetime of the universe:
This mystery, the end of the universe, might not keep you up at night, but it will certainly be of concern to beings alive far into the future. This epic event is predicted to occur in about 10 billion years. Two opposing theories are the Big Crunch and the Big Rip. Neither of these outcomes sound terribly fun. The big crunch is the opposite of the Big Bang – all of the pieces of matter in the universe will stop accelerating away from each other and start accelerating toward each other. A boiling collision of all of the matter in the universe ensues (and mankind is unlikely to survive that). The Big Rip is where all of the pieces of matter in the universe continue to accelerate away from each other, faster and faster until eventually space-time moves so fast that it rips atoms apart (mankind is also unlikely to survive that one).
These two possibilities aren't the only possible outcomes for the universe – sadly it seems unlikely that our generation will ever know its fate.
Why can't we imagine four dimensions?
We little humans struggle to envision a world with four spatial dimensions. Some theories (such as string theory) need as many as eleven dimensions to be hypothetically possible. If string theory turned out to be correct, we'd have to figure out how there are six missing dimensions tangled up in our reality. I can feel a headache coming on...
Why does light have a universal speed limit?
c, the speed of light constant, is valued at 3x108 meters per second. But why this figure and not, for example, 4x1020 m/s? Is it a random digit pulled out of a bag of numbers when a new universe explodes into existance? It's currently impossible to know why the speed of light is the speed that it is... all we know is that our universe couldn't exist without this limit.
Unifying the big and the small:
Everything big, like stars and black holes, is made up of small things: particles. Einstein's laws of relativity govern the very big, while quantum mechanics is king in the realm of the very small. But physicists can't seem to jam the two theories together. The trouble is that gravity just doesn't appear to work on the nanoscopic scale. And bizarre quantum effects, like quantum tunneling (whereby an atom can "tunnel" through an otherwise impenetrable boundary), can't be applied to planets or stars. Your eyes would likely pop if the Moon suddenly "tunneled" through the Earth. It seems barmy that there would be one theory for everything big and another for everything small. Some scientists are trying to tackle this problem, and even making headway, but the missing link is still incredibly elusive.