https://www.scientificamerican.com/article/gravitational-waves-reveal-the-heart-of-neutron-stars/

Inside a neutron star—the city-size, hyperdense cinder left after a supernova—modern physics plunges off the edge of the map. There, gravity squeezes matter to densities several times greater than those found in the nucleus of an atom, creating what theorists suspect could be a breeding ground for never-before-seen exotic particles and interactions. But densities this high cannot be probed by laboratory experiments, and remain too challenging for even today’s most powerful computers to tackle.

So when the universe deigned to help out, astronomers jumped at the chance. Last August the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), along with a European detector named VIRGO picked up gravitational waves reverberating through spacetime from the merger of two neutron stars some 130 million light-years from Earth. Those waves, now reanalyzed in a new paper by the LIGO–VIRGO team, provide some of the best hints yet about the nature of the merger’s progenitors—and what neutron star stuff actually is.