The axion and the neutralino

And dark energy: Out There on NyTimes

"Which leaves abnormal matter, or what physicists call nonbaryonic matter, meaning that it doesn’t consist of the protons and neutrons of “normal” matter. What’s more (or, perhaps more accurately, less), it doesn’t interact at all with electricity or magnetism, which is why we wouldn’t be able to see it, and it can rarely interact even with protons and neutrons, which is why trillions of these particles might be passing through you every second without your knowing it. Theorists have narrowed the search for dark-matter particles to two hypothetical candidates: the axion and the neutralino. But so far efforts to create one of these ghostly particles in accelerators, which mimic the high levels of energy in the first fraction of a second after the birth of the universe, have come up empty. So have efforts to catch one in ultrasensitive detectors, which number in the dozens around the world."