To solve a long-standing puzzle about how long a neutron can ‘live’ outside an atomic nucleus, physicists entertained a wild but testable theory positing the existence of a right-handed version of our left-handed universe. They designed a mind-bending experiment to try to detect a particle that has been speculated but not spotted. If found, the theorized ‘mirror neutron’ — a dark-matter twin to the neutron — could explain a discrepancy between answers from two types of neutron lifetime experiments and provide the first observation of dark matter.