Large Hadron Collider: Dark matter's 'gang of four'
Next week, the Large Hadron Collider at Cern will be fired up again after a two year programme of maintenance and upgrading.
When it is, the energy with which it smashes particles will be twice what it was during the LHC's Higgs boson-discovering glory days.
It is anticipated - hoped, even - that this increased capability might finally reveal the identity of "dark matter" - an invisible but critical entity that makes up about a quarter of the Universe.
BBC Horizon has been speaking to scientists who helped develop our best theories of dark matter.