On 16 Aug 1999 14:28:15 +0200 Anders Sandberg <firstname.lastname@example.org> writes:
...locally (i.e. the known universe) there is much more matter than
> antimatter - why is one of the big questions in cosmology.
It is clear that the Moon, and various other planets to which we have sent probes, are not made of anti-matter, since the probes did not annihilate upon contact. But I don't know what specific indication we would have as to whether some distant galaxy is made of matter, or anti-matter. In general it seems that if there were substantial amounts of anti-matter laying about in the universe that we might see frequent energetic annihilations.
> Every kind of particle has a corresponding antiparticle - electrons
> have positrons, protons antiprotons, quarks have antiquarks etc. The
> exceptions are those particles that have themselves as
> antiparticles, such as photons.
> Anders Sandberg
Thanks for clarifying the case of photons, since I had never heard of an anti-photon. A photon reportedly has no rest mass, yet it behaves as if it has mass in the sense that it has momentum and is attracted by a gravitational field. I presume that if there were enough energy in the form of photons concentrated at one locality, a black hole could form, trapping the photons in their own gravity, just as a black hole can form if there is enough cold mass in one locality.
Keith Lynch has already stated that there is no such thing as negative mass in the sense of mass which has a gravitational repulsion instead of attraction. So a neutron would have a gravitational attraction to an anti-neutron just the same as would a neutron to a neutron.
Space is full of large numbers of neutrinos flying around, and some of them may be from distant galaxies. The neutrinos seldom interact with matter, but there are so many of them that neutrino detector tanks can occasionally detect them. It was recently reported in Scientific American that there is evidence that neutrinos may have mass, so that raises the question of whether there are antineutrinos and what would happen if an antineutrino had an interaction in one of the neutrino detectors.