why there is more matter than antimatter...

While there may be some form of matter with a negative mass value/charge, the current definition of antimatter does not include that property. The idea of antimatter arose out of solutions ( by Paul Dirac 1926 ) of quantum mechanical equations. Some solutions were the familiar particles already known, but others were a new form that would have the property of annihilating with the corresponding known particles. In a rough sense they are a 'negative' solution, although you have to be a bit careful about what you mean by that. The charge is also inverted.

So an electron with a charge that we designate as negative has the anti-particle version called the positron which is positively electrically charged. Both have positive mass ( inertial and gravitational ) and thus respond in the same way gravitationally and dynamically. A number of fine experiments have demonstrated that. A proton, with positive charge, has it's anti-proton. The neutron has it's anti-neutron. If either particle meets it's antiparticle both are replaced by energy in say photon form. So a neutron meeting an anti-neutron, both which have no electric charge, will annihilate. Mind you a proton meeting with a positron won't annihilate even though they are matter and anti-matter - they'll respond according to other characters though, so you'd expect electrical repulsion as they both have a positive electrical charge.

Perhaps a better way of viewing this is that all particles now have an extra ( quantum ) number associated with them that records whether they are matter or antimatter. Plus for matter, negative for antimatter. Note this is not a recording of electric charge. The force 'carriers' are zero on this scale, that is there is no anti-particle for a photon, say. For any system of particles the total of this number over all of them is a constant at all times. So when an electron ( +1 ) meets a positron ( -1 ) the photons ( each 0 ) are emitted during annihilation [ thus (+1) + (-1) = 0 + 0 ]. So you have a conservation or invariant principle here. Without going into deep explanations, which have some surprising convolutions, this conservation is the result of underlying symmetries. Richard Feynman characterised the anti-particle of 'X' as the particle 'X' but moving backward in time. Hmmmm .......

Well, almost symmetrical. Indeed one popular/favoured reason why matter dominates numerically over anti-matter today is some issue of symmetry 'breaking' during the early universe. This refers to single 'simple' symmetry operations breaking but not a combined group of operations called CPT - for Charge ( electric ), Parity ( mirror reflection ) and Time. To date the CPT symmetry combination is upheld by experiment, so no known exceptions. Yet. The breaking means that not all, but some, interaction situations do not uphold the conservation laws alluded to above. These have been observed at 'low' energies ( in 50's and 60's particle accelerators ) where non-equal proportions of decay products are formed - where full adherence to each symmetry separately would predict equal amounts. Like most quantum mechanical stuff, we are applying statistical arguments to groups of results and not a specific prediction of one event.

Now consider really high energies, such as the unimaginably high temperatures shortly after the Big Bang. One set of mechanisms has been applied to predict unequal decay of 'stuff' to matter/antimatter. The antimatter that is produced annihilates with an equal amount of matter for sure, but there will be matter left over. If you have more boys than girls at a dance then after all the girls have paired off with a boy each, some boys will be hanging around partner-less. We are made of those partner-less particles. You can produce antimatter these days, but only with the corresponding matter to boot ie. not in excess. Also of note here is that force particles ( say photons ) numerically outweigh matter particles by about a factor of a billion to one. Well, in the universe that we can observe/access at least. These photons are probably the products of the initial orgy of annihilation.

A quite terrific book about all of this, that I would recommend, is by Leon Lederman ( a Nobel Laureate ) called 'Symmetry and the Beautiful Universe'. It has virtually no maths in the main text, but you still have to concentrate though, as some conclusions can sneak up on you! :-)

Cheers, Mike.

( edit ) The quantum number that denotes matter/antimatter is the baryon number, where 'bary' means 'weight' or 'heavy' ie. has ( rest ) mass. I'm politely not mentioning the intermediate vector bosons, Higgs etc.

Could be either, to fit the hypothesis of unseen but yet gravitationally acting material ( positive mass and attractive ). If it was antimatter it would have to be sufficiently separate from ordinary matter to diminish the hubbub we'd otherwise see as annihilation occurs. Matter/antimatter annihilation is 100% efficient for mass to energy conversion and is thus quite 'bright' when it happens. So I suppose that's also why everyone wants their warp drives powered by it, the ultimate bang for your buck! :-)

You know what I really see as weird in wording or presentation, and that is the Dark Energy business driving the universe apart. Now if any energy is equivalent to some mass then why can't I say that we have 'mass with repulsive properties' pushing stuff apart. For me at least, I have certainly lost the thread of definitions on that topic. I grant that the acceleration of cosmic expansion is probably real but I think the GR explanation ( all that minus one-third of the negative pressure stuff ) has been pushed too far. Sign a missing puzzle piece.

Cheers, Mike.