The mysteries don’t end there. Atoms are known to be electrically neutral — the positive charge of the protons is cancelled out by the negative charge of the electrons — but as to why this is so, Lincoln says, “Nobody knows.”

2. Why is gravity so weird?

No force is more familiar than gravity — it’s what keeps our feet on the ground, after all. And Einstein’s theory of general relativity gives a mathematical formulation for gravity, describing it as a “warping” of space. But gravity is a trillion trillion trillion times weaker than the other three known forces (electromagnetism and the two kinds of nuclear forces that operate over tiny distances).

One possibility — speculative at this point — is that in addition to the three dimensions of space that we notice every day, there are hidden extra dimensions, perhaps “curled up” in a way that makes them impossible to detect. If these extra dimensions exist — and if gravity is able to “leak” into them — it could explain why gravity seems so weak to us.

“It could be that gravity is as strong as these other forces but that it gets rapidly diluted by spilling out into these other invisible dimensions,” says Whiteson. Some physicists hoped that experiments at the LHC would give a hint of these extra dimensions — but so far, no luck.

3. Why does time seem to flow only in one direction?

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Since Einstein, physicists have thought of space and time as forming a four-dimensional structure known as “spacetime.” But space differs from time in some very fundamental ways. In space, we’re free to move about as we wish. When it comes to time, we’re stuck. We grow older, not younger. And we remember the past, but not the future. Time, unlike space, seems to have a preferred direction — physicists call it the “arrow of time.”

Some physicists suspect that the second law of thermodynamics provides a clue. It states that the entropy of a physical system (roughly, the amount of disorder) rises over time, and physicists think this increase is what gives time its direction. (For example, a broken teacup has more entropy than an intact one — and, sure enough, smashed teacups always seem to arise after intact ones, not before.)

Entropy may be rising now because it was lower earlier, but why was it low to begin with? Was the entropy of the universe unusually low 14 billion years ago, when the Big Bang brought it into existence?

For some physicists, including Caltech’s Sean Carroll, that’s the missing piece of the puzzle. “If you can tell me why the early universe had a low entropy, then I can explain the rest of it,” he says. In Whiteson’s view, entropy isn't the whole story. “To me,” he says, “the deepest part of the question is, why is time so different from space?” (Recent computer simulations seem to show how the asymmetry of time might arise from the fundamental laws of physics, but the work is controversial, and the ultimate nature of time continues to stir passionate debate.)

4. Where did all the antimatter go?

Antimatter may be more famous in fiction than in real life. On the original Star Trek, antimatter reacts with ordinary matter to power the warp drive that propels the U.S.S. Enterprise at faster-than-light velocities. While warp drive is pure fiction, antimatter is very real. We know that for each particle of ordinary matter, it's possible to have an identical particle with the opposite electrical charge. An antiproton is just like a proton, for example, but with a negative charge. The antiparticle corresponding to the negatively charged electron, meanwhile, is the positively charged positron.

Physicists have created antimatter in the laboratory. But when they do, they create an equal amount of matter. That suggests that the Big Bang must have created matter and antimatter in equal quantities. Yet almost everything we see around us, from the ground beneath our feet to the most remote galaxies, is made of ordinary matter.

What’s going on? Why is there more matter than antimatter? Our best guess is that the Big Bang somehow produced a tiny bit more matter than antimatter. “What had to have happened early in the history of the universe — in the very moments after the Big Bang — is that for every 10 billion antimatter particles there were 10 billion and one matter particles,” says Lincoln. “And the matter and the antimatter annihilated the 10 billion, leaving the one. And that little ‘one’ is the mass that makes up us.”

But why the slight excess of matter over antimatter in the first place? “We really don’t understand that,” Lincoln says. “It’s bizarre.” Had the initial amounts of matter and antimatter been equal, they’d have annihilated each other completely in a burst of energy. In which case, says Lincoln, “we wouldn’t exist.”