Neutron stars are made of neutrons. It’s not an element that you could put on the periodic table, if that’s what you mean. So, no to the neutronium.

Yes, neutron stars can turn into black holes, if they aquire enough mass to overcome the neutron pressure. We see this in compact binaries where one of the members is a neutron star.

The first stars formed out of the much denser early universe. Technically, a galaxy is a collection of billions of stars, so the first stars formed from the hydrogen gas clouds before there were galaxies.

You’re last question is a little beyond my knowledge and really hasn’t been figured out, yet. Dark energy is the term for what is causing the acceleration of the expansion of the universe. It is acting on space itself and not matter, as far as we know. But again, this is still new and not well understood territory.

Love all the questions!