Last month I discussed the experiments using vast tubs of liquid xenon to search for dark matter. It turns out to have been a prescient post, because yesterday the scientists from LUX (a liquid xenon experiment buried deep under the Black Hills in South Dakota) posted their latest results. And they saw.... absolutely nothing. LUX is the most sensitive experiment of its kind, and these scientists were able to push down upper limits on how strongly the dark matter interacts with ordinary matter by a factor of 4.
So why haven't we detected dark matter yet? It might still be hiding just below the current sensitivity of our experiments. But it's also possible that we're barking up the wrong tree.
These xenon experiments are most sensitive to dark matter particles about a hundred times more massive than a proton -- the mass of a typical atomic nucleus. But it's possible that the mass of the dark matter particle is much smaller or much larger than this. A more depressing possibility is that the dark matter doesn't interact with ordinary matter at all, except through gravity. That would make it almost completely undetectable in any of our current experiments. And yet another idea, given new life by the recent LIGO discovery of colliding black holes, is that maybe the dark matter is composed of black holes left over from the early universe.
And is it possible that there's no dark matter at all? Dark matter is really a failure of gravity to behave the way it should on galaxy-sized length scales. So what if our theory of gravity were wrong? This idea has long be a subject of speculation, but it was dealt a serious blow by the observation, a few years ago, of the Bullet Cluster -- a system of two colliding clusters of galaxies. The ordinary matter in the two clusters has pancaked, like the opposing linemen colliding in a football game, but there is clearly some sort of matter that has slipped right through the "linemen" on both sides of the cluster -- that's exactly what you'd expect if there were dark matter present, and it's almost impossible to explain by modifying gravity. It looks like Einstein got it right the first time.