Neutron stars are the unimaginably dense remnants of collapsed giant stars. They get their name because the conditions inside are so fierce that atoms are smashed apart into a soup of protons, electrons, and, yes, neutrons. And now we have the first direct evidence that neutron stars are forming superfluids of neutrons - a totally bizarre state of matter that can't even be created in Earth laboratories.
A superfluid is sort of like a liquid, except its behavior can be very strange. Basically, a superfluid is where viscosity drops to zero and thermal conductivity becomes infinite, the upshot of which is the superfluid flows uncontrollably in all directions while maintaining the same temperature throughout. Even gravity is no longer a barrier for superfluids - it can flow right up the side of a beaker and escape. Superfluids essentially live in a world without friction.
Now, in a neutron star, superfluids can only form when neutrons pair up. That process should have certain telltale signs, including the steady release of neutrinos and a resultant drop in the energy, and thus brightness, of the star. The neutron star formed from the remnants of the Cassiopeia A supernova fits this bill perfectly - since its discovery in 1999, the neutron star has lost 20% of its brightness and about 4% of its temperature. That's incredibly rapid temperature loss, and the best explanation for it is the creation of neutron superfluids inside the star.
Superfluid neutrons are one form of matter we're unlikely to see in a laboratory any time soon. Particle accelerators can create matter of the required density, but only at enormous expense of energy, which means the temperature of the matter would be far, far higher than that inside a neutron star. That significantly greater temperature makes the creation of a neutron superfluid impossible in laboratory conditions. Until we can learn to replicate the high density, relatively low temperature conditions inside a neutron star - which isn't likely to happen anytime soon - this is one form of matter that will have to remain exclusively out in the cosmos.