Could electrons be a form of antimatter? I've played with this idea for years, and haven't really been able to eliminate it. So, perhaps someone here can point to simple experimental evidence that would do so.
Here's the issue: Antimatter of course annihilates ordinary matter, but the more precise statement is that antiparticles annihilate the same types of particles.
That means that you can mix antimatter with matter without it going boom, as long as you only allow one of each type in the mix. For example, positrons (anti-electrons) can pass through interior of a neutron at low speed without causing annihilations for the simple reason quarks and electrons cannot pair off for annihilation.
All of which leads to this delightfully non-standard line of thought: If what we call electrons were actually anti-particles, would we even be able to tell?
A more precise way to express that is this: If all negatively charged particles had the internal chirality of antimatter, and all positively charged particles that of matter, would the resulting transformed theory lead to paradoxes or contradictions with the Standard Model? Or would it instead produce an experimentally isomorphic theory that describes what we see just as accurately as the Standard Model?

(The neutrinos of course have no charges, so their affiliations would be determined by their associations with the electron types.)

So my question is this: Would reclassifying the chirality of particles so that "pro" always means positive particles and "anti" always means negative particles result in a version of the Standard Model that is experimentally isomorphic to known results, or would it instead produce one or more clear contradictions or paradoxes?
One final note that's too much fun not to mention: a workable pro="+"/anti="-" theory would convert the mystery of why matter dominates over antimatter into a mystery of the particle mix came out so asymmetrically.
 A: There is some freedom in deciding which particle in a particle-antiparticle pair is called "matter" and which is called "antimatter" but the freedom is smaller than you think. A basic problem is that your sentence

Antimatter of course annihilates ordinary matter, but the more precise statement is that antiparticles annihilate the same types of particles.

isn't really true. In fact, the opposite statement, while inaccurate, is much closer to the truth: matter and antimatter often can annihilate even if they belong to different species.
For example, a proton will rapidly annihilate with an antineutron (or an up-quark with anti-down-quark, if we look at the same process at the quark level), leaving some positron and neutrino (whose rest mass is much lower than the rest mass of either proton or antineutron) with lots of energy.
The up-quark and down-quark are different species or flavors but it would make no sense to call one of them "matter" and the other "antimatter" because they can "almost annihilate" to "almost nothing". More generally, all quark flavors are similar and it's better to call all of them "matter", especially because they may be related by symmetries that don't have a reason to include charge conjugation C.
Now, the atoms are composed of protons and electrons – and we call the atomic bound states "matter". That implies that an electron has the same "pro-anti" label as the six quarks. There are no bound states between positrons and protons so there would be no atomic "matter" if you flipped the convention for electrons but not protons.
Grand unified theories actually do link some 2-component spinors to larger representations and these representation contain fields that create both matter and antimatter so the binary label "pro-anti" becomes more subtle in such theories. We must still carefully distinguish a field and its Hermitian conjugate.
The "pro-anti" dichotomy remains meaningless for some particles, anyway. There are totally neutral particles – photons, Z-bosons, gluons, gravitons – that are identical to their antiparticles so here there is no "polarization", of course. There are also charged particles, W-bosons, for which it makes no sense to ask which of them is matter and which of them is antimatter. A W-boson may decay to a quark-antiquark pair so it's equally "far" from matter as it is from antimatter.
Neutrinos seem to be Majorana particles so far so they are identical to their antiparticles, too. However, the helicity (left-handed, right-handed) is correlated with the usual labels "pro-anti" which means that we can distinguish matter from antimatter, after all. There can also be right-handed neutrinos in which case the separation of neutrinos to "matter" and "antimatter" is exactly as possible (in principle) as it is for electrons and positrons.
A: Short answer: the electron “goes with” the quarks that make up the proton and neutron.  The “weak” interaction between u and d quarks is the same one that works between the electron and neutrino.
The preference of the weak interaction for left handed particles (or right handed for antiparticles) tells you that electrons are matter.
There are concerved numbers that can be found to balance nicely if you categorize electrons as matter.
A: The term anti-matter was not invented by particle physicists, but by science fiction authors, so there is no point in studying the Standard Model to see how it defines the term. Moreover, particle physics does not even seek to define the meaning of the term matter, without which there can be no logical meaning to its opposite, anti-matter.
Einstein proposed that particle physics should be based on the energy equivalence principle:  thus matter as a concept has no meaning, since particles consist entirely of energy, albeit in a state of confinement. Release that energy from its confined state, and you have a Hiroshima-level of energy liberation.
The classical science fiction depiction of anti-matter suggests some very similar large type of explosion, if matter and anti-matter meet. But it is more logical to regard a particle and its anti-particle as neutralising one another. A particle confines a huge quantum of energy, but has a relatively tiny electric charge: if it encounters an anti-particle the two will behave in essence like any pair of charged particles: +1 applied to -1 will sum to zero.
What we ought to be considering is the mechanism by which a particle and its anti-particle differ from one another. If the difference turns out to be a question of spin, for example, one could see how two opposed particles (i.e. having opposite spin) could meet and in effect neutralise one another, without necessarily disturbing the confinement of the energy bound up within them.
It is evident that most members of the particle zoo do not lend themselves to a neat division between matter and anti-matter. What modern physics needs is to avoid any such old-fashioned terms, which belong firmly to classical 19th Century thinking. The notion that the universe can be divided into two opposed camps flies in the face of everything we have discovered in the past hundred years: the electron has an opposite charge to the proton, but no scientist proposes to label the electron as anti-matter. And when the electron and proton meet, they co-exist - they do not mutually annihiliate, there is no explosion; they sit together, and merely cancel each other out (in terms of their opposed electric charge).
We cannot yet see a quark, or an electron: they are billions of times smaller than the best current visual aid - the electron microscope - can magnify. But only by seeing them will we determine why a quark differs from an anti-quark.
What we do know is that a high energy x-ray laser is capable of splitting an electron into three distinct particles: one carrying its charge, one its spin, and the other its orbital motion. So we should be careful not to be "sure" that an electron represents anti-matter, when we are not even sure that we really understand what an electron is. Clearly, it is not a particle: it is at best a combination of three particles; just as a proton is not a particle, but merely three quarks in combination.
Now that we have the beginnings of a means for studying the particle which gives the electron charge, we are one step nearer to an understanding of what causes charge, which can help us understand what gives charge to a quark, and why many particles in the particle zoo don't have charge at all.
Perhaps that may explain why some quarks can anomolously lack charge, causing their proton to exhibit negative charge (so-called anti-matter), or how an electron can exhibit positive charge (again, so-called anti-matter).
We are assured by no less an authority than Einstein that so-called matter is really a combination of energy and the square of the speed of light: so we know that particles are really a field of energy, not a solid object. It is a small step to then suggest that particles may be sub-divisible -
as the electron is now seen to be.
The quark must also be divisible into sub-particles, one of which carries its charge: most likely the same sub-particle as carries the electron's charge. Some packet of energy, responsible for what, at a larger scale, we perceive as a difference between matter and anti-matter.
