Of all the possible states of matter - dark matter, negative mass - this is the one that really brings on the nothing. Supermassive particles with no charge whatsoever. And they're theorized to help jumpstart cold fusion.
Neutrium is also known as neutronium. It was first officially, scientifically given a name by JC Fisher, in a paper about polyneutron chain reactions. Fisher, also in this paper, coined the term 'polyneutrons.' Neutrons don't tend to group together all on their own. Although every atom heavier than hydrogen has some neutrons in it, they just don't aggregate into 'nuclei' of their own. The term 'nuclei' is problematic, since the neutrons would have no charge with which to attract electrons and so wouldn't really be atoms as we define them. Even a system made up of two neutrons isn't bound together the way the particles in a regular nucleus would be. The two neutrons feel an attraction to each other and can be pushed into each other, they don't coalesce into a nucleus. Two neutrons together are the biggest group of neutrons anyone has reliably reported, and they're short-lived products of radioactive decay. Four neutrons together has been theorized as possible, but nothing more extensive than that. Polyneutron chains, as Fisher conceives of them, would be anything from four to four thousand neutrons all stuck together.
Since Neutrium would not have any electrons, it also wouldn't have many chemical properties normally associated with atoms. Fisher's theory was that since Neutrium wouldn't mix with anything, and so a few particles of it could simply stay in anything, even ordinary water. Although Neutrium wouldn't participate in ordinary chemical reactions, it still could tunnel into nuclei and kick off nuclear ones. Therefore, Neutrium, in properly prepared water, might start cold fusion reactions and be a power source.
It is a pleasant idea, an exotic particle that might help us solve our energy woes, but it's unlikely. No one has ever seen a polyneutron, and although science fiction holds that they're in the center of neutron stars, most scientists think that there aren't really even neutrons in the middle of neutron stars. Matter there is probably broken down even farther, into quarks and subneutrons, and is generally referred to as 'neutron-degenerate matter.' Still, we can hold out hope - and take a closer look at our water - and think about what might be. Something very, very neutral, no doubt.
Neutron Star Image: NASA/Dana Berry