You've probably never heard of this one before. Neutronium is a hypothetical substance made purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 for the hypothetical "element of atomic number zero" placed at the head of the periodic table.
However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars.

The symbol I chose for Neutronium, the astronomical symbol for stars, is a reference to those neutron stars. Possible unicode characters include: * ∗ ✱ ✶ ★ ⚹

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