As was recently reported in the New York Times and elsewhere, scientists working at Fermilab claim to have new evidence of a natural preference for matter over anti-matter. The paper can be found here.
The history of the argument goes like this: Every time physicists come up with a new equation, they try to see what happens when you change the sign, + to – or – to +, of some of its components. For distance, velocity, position and angle there is a physical interpretation to the switch, but for things like charge and mass and time it’s a little murkier. There are obvious examples of positive and negative charge (protons and electrons,) and anti-matter is a reasonably well understood interpretation of “negative mass,” but negative time (or a negative direction of time) is almost impossible to deal with practically, although Richard Feynman and others proposed theories as to what it means.
These instances of possible inversion are called symmetries, and there are three of note: Electric charge, Parity (often described as left- or right-handedness), and Time. Many physicists believe that the universe should behave identically if you invert all electric charges, parity, and the direction of time. This is called CPT symmetry. We can’t, as far as we know, reverse the direction of time, but we can examine cases of alternate charge and parity (just CP symmetry now) and look for differences.
This is important to physics because it might posit an explanation for why there is any matter in the universe. Ordinarily, when you create particles (E=mc²) you have to do so in equal amounts of matter and anti-matter due to conservation laws. This would either forbid any stable collection of material or require there are anti-planets and anti-stars out there in the universe, which we would almost certainly know about through the tremendous amounts of radiation released by matter/anti-matter collisions in interceding space. So far as we can tell, the universe is just about all matter.
There have been experiments done showing CP symmetry violation, but the events involved are not capable of producing enough matter to account for what we see. This recent experiment might not be either, but it dramatically strengthens the argument that this universe prefers our stuff to its inverse.