Abstract: New states that mix with the neutron, such as dark baryons and mirror neutrons, have been proposed to address dark matter, baryogenesis, the long-standing neutron lifetime anomaly, and the recent XENON1T excess. I show that such states are extensively probed by (1) Big Bang nucleosynthesis, (2) cosmic microwave background spectra, (3) stability of nuclides in low-metallicity stars, (4) a novel mechanism to heat neutron stars by tapping the energy stored in their Fermi seas. This latter effect already constrains a vast range of parameter space, and provides a powerful motivation for imminent optical, ultraviolet and infrared astronomical missions, namely, of directly probing the properties of the neutron. Talk based on 2012.09865 & 2105.09951.