Quark matter – an extremely dense phase of matter made up of subatomic particles called quarks – may exist at the heart of neutron stars. It can also be created for brief moments in particle colliders on Earth, such as CERN’s Large Hadron Collider. But the collective behaviour of quark matter isn’t easy to pin down. In a colloquium this week at CERN, Aleksi Kurkela from CERN’s Theory department and the University of Stavanger, Norway, explained how neutron-star data have allowed him and his colleagues to place tight bounds on the collective behaviour of this extreme form of matter.