It's a big mystery why anything exists.
I'm not just being philosophical, here. According to the current most widely-accepted cosmological model, when the Big Bang occurred, matter and antimatter would have formed in equal quantities. As anyone who has watched Star Trek knows, when matter and antimatter come into contact, they mutually annihilate and all of the mass therein is converted to a huge amount energy in the form of gamma rays, the exact quantity of which is determined by Einstein's law of E = mc^2.
So if we started out with equal amounts of matter and antimatter, why didn't it all eventually go kablooie, leaving a universe filled with nothing but gamma rays? Why was there any matter left over?
The answer is: we don't know. Some cosmologists and astrophysicists think that there may have been a slight asymmetry in favor of matter, driven by random quantum fluctuations early on, so while most of the matter and antimatter were destroyed by collisions, there was a little bit of matter left, and that's what's around today. (And "a little bit" is honestly not an exaggeration; the vast majority of the universe is completely empty. An average cubic meter of space is very unlikely to have much more than an atom or two in it.)
One question this sometimes brings up is whether the stars and galaxies we see in the night sky are matter; if, perhaps, some entire galaxies are made of antimatter, and there really are equal amounts of the two. After all, antimatter is predicted to act exactly like matter except that its fundamental particles have the opposite charges -- its protons are negative, its electrons positive, and so forth. So a planet entirely formed of antimatter would look (from a safe distance) exactly like an ordinary planet.
And just to throw this out there, an antiplanet wouldn't have copies of all of us except for having the opposite personalities, for example some people who are good guys being evil and/or having beards, as outlined in the highly scientific Lost in Space episode "The Antimatter Man:"
Nor would there be a creepy bridge between the two universes, covered with fog and backed by eerie music:
Which is a shame, because I always kinda liked that episode.
Considerations of evil Major Don West with a beard notwithstanding, here are two arguments why most physicists believe that the stars we see, even the most distant, are made of ordinary matter. The first is that there is no known process that would have sorted out the matter from the antimatter early in the universe's life, leaving isolated clumps of each to form their respective stars and galaxies. Secondly, if there were antistars and antigalaxies, then there'd be an interface between them and the nearest clump of ordinary stars and galaxies, and at that interface matter and antimatter would be constantly meeting and mutually annihilating. This would produce a hell of a gamma ray source -- and we haven't seen anything out there that looks like a matter/antimatter interface (although I will return to this topic in a moment with an interesting caveat).
A paper last year found that the key to understanding why matter prevailed might lie in the mysterious "ghost particles" called neutrinos. There are three kinds of neutrinos -- electron neutrinos, muon neutrinos and tau neutrinos -- and one curious property they have is that they oscillate, meaning they can convert from one type to another. The rate at which they do this is predicted from current theories, and it's thought that antineutrinos do exactly the same thing at exactly the same rate.
The experiment described in the paper took place in Japan, and found that there is an unexpected asymmetry between neutrinos and antineutrinos. Beams of muon neutrinos and muon antineutrinos were sent on a six-hundred-kilometer journey across Japan, and upon arriving at a detector, were analyzed to see how many had converted to one of the other two "flavors." The surprising result was that the neutrinos had oscillated a lot more than predicted, and the antineutrinos a lot less -- something called a "CP (charge-parity) violation" that shows antimatter doesn't, in fact, behave exactly like matter. This asymmetry could lie at the heart of why the balance tipped in favor of matter.
But now a new analysis of data from the Fermi Gamma-ray Space Telescope has thrown another monkey wrench into the works. The study was undertaken because of a recent puzzling detection by an instrument on the International Space Station of nuclei of antihelium, which (if current models are correct) should be so rare in the vicinity of ordinary matter that they'd be entirely undetectable. But what if the arguments against antistars and antigalaxies I described earlier aren't true, and there are such odd things out there? Antistars would be undergoing fusion just like the Sun does, and producing antihelium (and other heavier antielements), which then would be shed from the surface just like our Sun sheds helium. And some of it might arrive here, only to fall into one of our detectors.
But what about the whole gamma-rays-at-the-interface thing? Turns out, the study in question, the subject of a paper last week in the journal Physical Review D, found that there are some suspicious gamma-ray sources out there.
Fourteen of them, in fact.
These gamma-ray sources are producing photons with an energy that's hard to explain from known sources of gamma rays -- pulsars and black holes, for example. In fact, the energy of these gamma rays is perfectly consistent with the source being ordinary matter coming into contact with an antistar.
Curiouser and curiouser.
It doesn't eliminate the problem of why the universe is biased toward matter; even if these are antistars, their frequency in the universe suggests that only one in every 400,000 stars is an antistar. So we still have the imbalance to explain.
But it's a strange and fascinating finding. Astrophysicists are currently re-analyzing the data from every angle they can think of to try and account for the odd gamma-ray sources in any way other than it being evidence of antistars, so it may be that the whole thing will fizzle. But for now, it's a tantalizing discovery. It brings to mind the famous quote from J. B. S. Haldane -- "The universe is not only queerer than we imagine, it's queerer than we can imagine."