Snideness aside if I were to think out loud (in text) we have a few clues to work with to describe negative matter. The first obvious clue is evidence of gravity and the second is absence of light
(invisibility). What is surprising to most people will be that we are still in the process of discovering what light is. You might say light is a photon but thats not the whole story. Great Physicists have step by step tried to unravel what light is.
What I would ask of dark matter/neg mass theory is under what conditions is there NO LIGHT, no interaction of photons with this unknown substance.

Let me spend 10 minutes talking about who and what we know about light before I talk about conditions of where and why light does not exist. I see photons as discrete tiny energy packets that do not interact with other photons but instead interact with other 'stuff'.

HISTORY OF THE PHOTON

Since Descartes, several scientists have studied this phenomena and have become more curious about light. This curiosity has led to discoveries about the physical properties of light.. One popular property of light is that it is electromagnetic radiation. Another property is that it can be quantized. When light is quantized, it is called a photon. Huygens, Newton, Hooke, Young, Fresnel, Faraday, Maxwell, Hertz, Planck, Lewis, Einstein, Lenard, Schrodinger, De Broglie, and Feynman have all contributed to the modern theories of light.

These are th contributions of each one; (you may want to skip this part)
Huygens explained how wave theory accounts for geometrical optics. Geometrical optics describes light propagation in terms of "rays".

Newton”s theory of light was based on Descartes. Though, Descartes was responsible for discovering the mechanical properties of light; he believed in corpuscularism. This philosophy discusses corpuscles ( or particles) and their interaction. Newton took Descartes’idea and developed the corpuscular theory of light. This theory states that light is a stream of particles.
Hooke investigated the phenomenon of refraction.Refraction is the phenomenon of light or radiation being deflected as it passes obliquely through the interface through a mediums of varying density.He suggested that light's vibrations could be perpendicular to the direction of propagation. A quantum system called “Hooke’s atom”is named for him. It is an artificial helium-like atom. In this system, the coulombic electron-nucleus interaction potential is replaced by a harmonic potential. The harmonic potential used to describe the electron-nucleus interaction is a consequence of Hooke's law. Hooke’s law states that the stress applied to an object is proportional to the strain produced.

Young studied the wave theory of light. He proved that the property of interference existed in waves of light. This was done by doing the double slit experiment. This experiment displays the wave-particle nature of matter. In regards to interference, the property of superposition is displayed. This property affirms the existence of entanglement.Entanglement involves properties of quantum states correlating with each other in quantum systems.

Fresnel was responsible for diffraction. Diffraction is the process by which a beam of light or radiation is spread out as a result of passing through either a narrow aperture or edge. This is accompanied by interference between the waveforms produced.

Faraday also established that magnetism could affect rays of light and that there was an underlying relationship between the two phenomena.

Maxwell proposed that light was a product of the interaction or entanglement of the electric and magnetic fields. An electric field is a region around a charged particle or object within which a force would be exerted on other charged particles or objects. Magnetic fields are produced by electric currents. It is an area around a moving electric charge within which the force of magnetism acts. An insight to this is that-if the photon, light or a quantum of electromagnetic energy, is a product of two fields interacting then other particles would be products of the interactions of other fields.

Planck discovered the concept of blackbody radiation. Blackbody radiation is the electromagnetic radiation that is radiated from an ideal black body. A black body is an object that is capable of absorbing all of the electromagnetic radiation that falls on it. The distribution of energy in the radiated spectrum of a black body depends on temperature .

Gilbert Lewis , who created the lewis dot structures for chemistry, also studied photochemistry. Photochemistry is the branch of chemistry concerned with the chemical effects of light. Because of this, he coined the word “photon” A photon is a particle representing a quantum of light or other electromagnetic radiation. It carries energy proportional to the radiation frequency but has zero rest mass. It is also an elementary particle and a force carrier. A force carrier is the product of forces between particles arising from the exchange of other particles.

Einstein is credited for the photoelectric effect. The photoelectric effect happens when electrons are emitted from matter. The electrons are emitted because the matter absorbs energy from electromagnetic radiation. This electromagnetic radiation has a short wavelength.

Lenard, through experimenting with cathode rays, discovered that the energy of the electron depends on the wavelength of light.

Schrodinger studied that light was coherent. This proves that light has the property of superposition.

De Broglie used the concept of frequency in mathematical relations to define wavelengths. There is another type of frequency. This is called angular frequency. Angular frequency is also known as angular speed. It measures the rotation rate similar to the twist in orbits or shape of DNA.

Feynman studied how light and matter interacted. He created the theory of quantum electrodynamics. This a quantum field theory that deals with the electromagnetic field and its interaction with electrically charged particles. He created a new notation to describe the interactions.
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Now what the heck is the (no light) invisible mass world and where did it come from". I propose it came from the anti matter - matter EXPLOSION that created Energy release, Higgs Field production and lots of "residue". For every billion parts of matter only one part of our matter was left intact. If 99% became radiation and just 1% became an elementary residue - thats alot of residue comared to our positive mass. I'll take 1% of a billion dollars.

I can only guess that this residue is something like fractured quarks. I do not think they are particles as we know it like anti protons McGrath.

I would add that it would be incorrect to think 2kg. of negative matter would weigh minus 2 kg.. My thoughts on this strange matter goes father but this already too long for now.