Neutronium: material that exists only in neutron stars.

"In theory, any type of baryon, or entity made of three quarks, can bind together to any other type of baryon. However, while protons-and-neutrons can bind together to form stable bound states (like atomic nuclei), neutrons-and-neutrons and protons-and-protons do not. " (BigThink, Ask Ethan: Why don’t neutrons bind together?)

Neutronium is a material that exists only in neutron stars. Or, neutron stars are material that is as close to neutronium as possible. There are no electrons that orbit those neutrons and that means the neutron star is not exactly neutronium. "Real" neutronium requires electrons. During the supernova explosion, the energy impulse smashes the electrons into the atom's core forming the structure that is like a giant neutron. 

Theoretical neutronium would be a pack of neutrons or a "miniature neutron star". And electrons orbiting it. In some theories, high-power electromagnetic radiation like laser beams can press the electrons into an atom's shell and create a ball, where there are only neutrons. Then the system can put the high-energy electrons in orbit that structure. 

The problem is that when energy pumping ends the energy that comes out from the neutron ball pushes electrons away. If researchers use lasers to stabilize that structure sooner or later the energy level rises so high, that the lasers cannot pump energy into it. 

In a supernova explosion neutron star stores a massive energy load and the massive gravity keeps that thing in its form. If the strength of gravity is weaker than this energy load the neutron star releases electrons. Or when the structure expands the electrons will separate from those particles. And that forms the carbon star. 

There is massive gravitation. And outcoming energy forms a condition. That denies neutron decay. The neutron star itself doesn't produce energy. But plasma and other materials. That impacts its shell and causes a situation. There the energy at the neutron star's shell is at a higher level than inside it. The massive gravitation along high energy levels makes the existence of neutronium possible. 

The neutronium is a hypothetical material that includes only neutrons. The neutron star is close to that material because it includes only neutrons. However, the neutronium does not exist in other places than in those dense, hot, and heavy objects. And that means neutronium is a useless material. A teaspoon of this material weighs as much as the Earth.

Above: Hypothetically, it would be possible for identical particles, such as neutrons, to form bound states with one another if there’s enough binding energy to make it so. Unfortunately, the only place where this can occur for more than a minuscule fraction of a second is inside a neutron star, where the gravitational force adds additional binding energy to the (insufficient) nuclear binding energy through the strong nuclear force. A one-time detection of a tetraneutron state, dating back to 2016, was finally replicated in 2022, showing that dineutron and tetraneutron states can very temporarily exist outside of a neutron star. (BigThink, Ask Ethan: Why don’t neutrons bind together?) 

As we see there is the gravity that bounds neutrons together in a neutron star. But there is no weak nuclear force or electromagnetic bounds between neutrons if they form structures there are only neutrons. One of the reasons why that thing happens is this. The difference in energy levels between protons and neutrons makes energy or weak nuclear force move in the atom's nucleus. Without that flow, the weak nuclear force cannot bind particles together. 

In some models, researchers can put neutrons together by pressing more energy into another neutron than others. That thing will not work at least in long-term solutions. The problem is that energy flows to the lower energy level until the entirety has the same energy level. And in that moment all particles start to form standing waves between them. That destroys the structure. 

 If all particles are identical, that means. They have the same energy level and they oscillate at the same frequency. That thing forms a situation where all neutrons oscillate synchronously at the same time. That thing forms energy waves that destroy this structure. 

Below: Free  neutron decay. (Wikipedia, free neutron decay)

One of the reasons for stable neutronium is impossible in nature. Is the short lifetime of neutrons. Neutron exists for only about 15 seconds and then it decays. And deliver energy. That energy destroys the structure. 

The energy falls to the neutron star but it cannot break its structure even if it jumps out from the middle of it. The massive gravity keeps the neutron star in one form and the energy that this thing creates keeps those neutrons existing. But when a neutron comes out from the atom's core its lifetime is about 15 seconds. And the "neutron graphene" could exist only in very high energy levels. 

Theoretically is possible to create a 2D neutron structure by pressing neutron clouds from both sides using massive high-power radiation. But that thing requires very much energy and the only thing that can make that material is high-power antimatter systems that create antimatter-matter annihilation between some other 2D material layers. 

The annihilation vacuum bomb can create short-term neutronium. The system would be the ball. The annihilation happens between graphene layers and it could create so much energy that it can smash electrons into the atom's core. That turns all particles into neutrons. The neutron star is a very dense and hot object and gravity on that object is extremely strong. 

The annihilation reaction around the molecular clouds can have enough energy to press electrons into the atom's core. And that should make the structure. That is similar to a neutron star. The mini-neutron star's existence is from a couple of seconds to minutes. The problem is that the annihilation vacuum bomb will turn into radiation during that process. 

Or maybe high-power laser systems can press neutrons into that structure. The problem is that the neutron net or neutron graphene requires so much energy that the system is not suitable because it's so expensive to use. The neutron graphene would be the strongest structure in the universe. But the problem is how to stabilize it. In some very futuristic visions, the spacecraft has a special fusion or antimatter reactor that covers its entire shell. 

The fusion acts as the electromagnetic press that happens on both sides of the neutron structure. And there the neutron net can stay. The problem is that the high-power fusion must happen symmetrically around the neutron net. And the temperature in that system would be billions of degrees. So that thing is possible in the far future. 

https://bigthink.com/starts-with-a-bang/neutrons-bind-together/

https://en.wikipedia.org/wiki/Free_neutron_decay