An experiment designed to catch a few WIMPs, the particles that are meant to make up dark matter, has failed to detect any sign of them.
Galaxies should be flying apart. Although the Milky Way feels pretty stable from the inside, like most galaxies, it's on the move. Galaxies move through the universe, sometimes colliding with each other. Most also move in relation to themselves, spinning around their center. A solid structure in motion can keep each of its particle together, but galaxies are just big groups of stars held to each other by the relatively fragile bonds of gravity. They should be shaking off material like a wet dog shakes off droplets of water. And yet, after careful analysis, scientists found that the galaxies were holding together far more than they should be.
The remarkable cohesion that galaxies display has been chalked up to the existence of dark matter. It's a strange form of matter that stays primarily in the center of galaxies. It makes itself known only by its gravity. It can't be seen, and it hasn't been touched, and no single particle of it has been isolated. The latest attempt to isolate a piece of dark matter has recently failed.
Dark matter is thought to be made up of particles called WIMPs. Weakly Interacting Massive Particles are particles that don't emit or absorb any form of light - so they can't be seen. They do have mass, though, so they can exert a gravitational pull on other particles. The problem is, it's hard to measure the gravitational pull between two particles, one of which is impossible to see.
Since it's hard to measure the gravity of a WIMP, and impossible to see one, the only way to find traces of dark matter is to have a WIMP run smack into a particle of regular matter. That's what the Xenon100 experiment aimed to do. A large copper vat of Xenon gas was surrounded with photoreceptors. When a piece of dark matter ran into the Xenon, the photosensors would catch it and record it. To make sure the Xenon wasn't colliding with itself, the Xenon was made as cold as possible. To make sure it wasn't getting stimulated by cosmic rays or other background radiation the copper vat was buried about 1400 meters underground. The experiment was left alone for 100 days - and after the hundred days were over scientists announced that the amount of interactions did not exceed the level of background radiation.
It could be that WIMPs are so very WIMPy that they need a more sensitive particle than Xenon to have measurable interactions with. Hopes are high that the Large Hadron Collider will detect a few WIMPs, or that more refined experiments will find them in the future. But the negative result begs the question - are they out there? And if they aren't, what is keeping galaxies from ripping themselves apart?