There has been an idea that quark matter with roughly equal numbers of up, down, and strange quarks could be absolutely stable, the ground state of nuclear matter. Ordinary charged nuclear matter would be inhibited from converting to it (or being absorbed into it) by a strong potential barrier at its surface. However, if this were true neutron stars would actually be quark stars. There is some evidence that this isn't so, so the theory is not held to be very likely these days.