Dark Matter and Anti-Matter

The mirror unvierse

Every basic particle in the universe appears to have an associate particle called its anti-particle that shares several of the same uniqueness, but many other uniqueness are the opposite of those for the particle. For example, the electron has as its antiparticle the anti-electron. The electron and the anti-electron have exactly the equal masses, but they have exactly opposite electrical charges.
The common substance just about us appears to be "matter", but we usually produce antimatter in small quantities in high energy accelerator experiments. When a matter particle meets its antimatter particle they destroy each other completely, releasing the equivalent of their rest masses in the form of pure energy (according to the Einstein E=mc^2 relation). For example, when an electron meets an anti-electron, the two annihilate and produce a burst of light having the energy corresponding to the masses of the two particles.
Because the properties of matter and antimatter parallel each other, we believe that the physics and chemistry of a galaxy made entirely from antimatter would closely parallel that of our our matter galaxy. Thus, is conceivable that life built on antimatter could have evolved at other places in the Universe, just as life based on matter has evolved here. However, we have no evidence thus far for large concentrations of antimatter anywhere in the Universe. Everything that we see so far seems to be matter. If true, this is something of a mystery, because naively there are reasons from fundamental physics to believe that the Universe should have produced about as much matter as antimatter.
Dark matter is the general term for matter that we cannot see to this point with our telescopes, but that we know must be there because we see its gravitational influence on the rest of the Universe. Many different experiments indicate that there is probably 10 times more matter in the Universe than the matter that we see. Thus, dark matter is basically what the Universe is made out of, but we don't yet know what it is!
As one simple example of the evidence for dark matter is the velocity of rotation for spiral galaxies depends on the amount of mass contained in them. The outer parts of our own spiral galaxy, the Milky Way, are rotating much too fast to be consistent with the amount of matter that we can detect; in fact the data indicates that there must be about 10 times as much matter as we can see distributed in some diffuse halo of our galaxy to account for its rotation. The same is true for most other spiral galaxies where the velocities can be measured.
There are various candidates for the dark matter, ranging from ordinary matter that we just can't see because it isn't bright enough to more exotic particles that have yet to be discovered. There are some fairly strong arguments based on the production of the light elements in the Big Bang indicating that the majority of the dark matter cannot be ordinary matter or anti-matter, and thus that the majority of the mass of the Universe is in a form very different from the matter that makes up us and the world around us. If that is true, then the matter that we are made of is but a small impurity compared to the dominant matter in the universe as someone has put it, "not only are we not the center of the Universe, we aren't even made of the right stuff!" The nature of the dark matter is perhaps the most fundamental unsolved problem in modern astronomy.