A LETTER FROM THE LEAD SCIENTIST

THERE EXISTS A FUNDAMENTAL new form of matter in our universe, outweighing everything we can see five times over. Its gravity holds the Milky Way together and yet we cannot see it, as it neither shines nor absorbs light.

Not only invisible, it is a “ghost”, passing through solid matter as if it were empty space, such that 100 trillion particles fly through your body every day and less than a handful might collide with a single atom. This unknown entity is dark matter, and determining its nature is one of the most important quests in science this century. The search for dark matter involves international teams of engineers and scientists building detectors with tonnes of noble gas or ultrapure crystals to record the rare impact of collisions. The detectors must be shielded against the otherwise blinding radiation from space, so they are built far underground. I am involved with one such facility, called SABRE, built inside a gold mine in Stawell, Victoria.

Because of radioactive traces contained in all steel manufactured since atmospheric nuclear weapon tests were conducted mid last century, even the metal used to build dark matter detectors can blind them. To escape this problem, teams search for pre-WW2 steel from sunken battleships, protected from fallout by the watery depths. The hunt for dark matter has been going on patiently for years and yet, beyond one tantalising but still controversial claimed detection, nothing has been found.

The challenging nature of dark matter means that the world of science is exploring other possible search strategies. One of these, described in this issue, uses naturally occurring salt crystals formed over a billion years ago. Perhaps, researchers suspect, they may contain tiny bullet-like tracts made by collisions eons ago. The biggest scientific mystery around, it seems, may be solved with a pinch of salt. When hunting for ghosts it pays to keep an open mind.