I wonder why pure small neutral nuclei (e.g. only 4 neutrons instead
of 2 protons and 2 neutrons like in He) seemed not to exist. I know
that giant stars can transform in so called "neutron stars", that
consist only of neutrons.
These kind of single neutral nuclei normally should be more likely and
even more stable, because there is no electromagnetical repulsive
forces between each other like in normal nuclei. It is clear that
single neutrons are not stable and decay within about 10 Min., but
bounded neutrons should exist. In standardmodel of particle physics
all quarks are equal to the strength of the strong interaction. The
strong force do not distinguish between the flavor, so there is no
sensible reason why neutral nuclei should not exist.