So with the Higgs particle sighted and the gongs distributed, physics seems finally ready to move on. Unless the Higgs had remained elusive, or had turned out to have much more mass than theories predicted, it was always going to be the end of a story: the final piece of a puzzle assembled over the past several decades. But now the hope is that the Large Hadron Collider, and several other big machines and experiments worldwide, will be able to open a new book, containing physics that we don’t yet understand at all. And the first chapter seems likely to be all about dark matter.
Depending on how you look at it, this is one of the most exciting or the most frightening problems facing physicists today. We have “known” about dark matter for around 80 years, and yet we still don’t have a clue what it is. And this is a pretty big problem, because there seems to be more than five times as much dark matter as there is ordinary matter in the universe.
It’s necessary to invoke dark matter to explain why rotating galaxies don’t fly apart: there’s not enough visible matter to hold them together by gravity, and so some additional, unseen mass appears to be essential to fulfil that role. But it must be deeply strange stuff—since it apparently doesn’t emit or absorb light or any other electromagnetic radiation (whence “dark”), it can’t be composed of any of the fundamental subatomic particles known so far. There are several other astronomical observations that support the existence of dark matter, but so far theories about what it might consist of are pretty much ad hoc guesses.