How can we be certain of the existence of dark matter when none of our instruments can detect it? More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe’s expansion. It turns out that roughly 73% of the Universe is dark energy. Dark matter makes up about 23%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn’t be called “normal” matter at all, since it is such a small fraction of the Universe. Empty space is not ‘nothing’ and are much more certain what dark matter is not than what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. There are three separate lines of evidence that the majority of dark matter is not made of ordinary matter including protons, electrons and atoms. The theory of Big Bang nucleosynthesis, which very accurately predicts the observed abundance of chemical elements, predicts that matter accounts for around 4-5 percent of critical density of the Universe. In contrast, evidence from large-scale structure and other observations indicates that the total matter density is substantially higher than this. Large astronomical searches for gravitational micro-lensing. Planck showed that around five-sixths of the total matter is in a form which does not interact significantly with ordinary matter or photons.
There is no good reason to assume that all the dark matter in the universe is built out of one type of particle and these new dark-matter particles would essentially consist of heavy “dark protons” and light “dark electrons” They would interact with each other far more than other dark matter particles to form “dark atoms” that use “dark photons” to interact through a sort of “dark electromagnetism”, much as regular protons and electrons interact through photons in conventional electromagnetism to build the atoms making up the stuff of everyday life. If dark atoms are possible, they could react with each other for dark chemistry, much as regular atoms interact chemically.
The dark world might even be as diverse and interesting as the visible world but the interactions between dark protons and dark electrons could make them lose energy over time. As such, they might slow down enough to clump into flat disks around galaxies, just like regular matter does. In contrast, most dark matter apparently forms roughly spherical haloes around galaxies, stars and planets. This concept means galaxies would have two disks, one made of regular atoms and one of dark atoms, which is why the investigators call their idea the double-disk dark matter model. The double-disk dark matter idea is a novel twist on an intriguing concept — that the physics of dark matter might be as complicated and interesting as the physics of ordinary matter is known to be. The basic possibility of a dark force very similar to electromagnetism — a long-range force with positive and negative charges, Such a model implies dark radiation, dark magnetic fields, and a host of other interesting phenomena. But we only had one kind of dark-matter particle in our model; to go to the world of dark atoms and dark chemistry requires more kinds of particles. That's the direction the new papers are taking. The gravitational effects of a dark atom disk on stars in galaxies could eventually be detectable via the European Space Agency’s Gaia space observatory scheduled to launch in October this year which aims to map the movement of approximately 1 billion stars in the Milky Way. This can be how we might first detect this dark disk, Moreover, since this novel form of dark matter is expected to be much slower on average than regular dark matter, it should be more susceptible for capture by the Earth, by the sun, or other heavy celestial objects. Annihilation of this dark matter captured by the sun can result in neutrino fluxes, which can be measured directly by the IceCube Neutrino Observatory on the South Pole. In addition, the dark electrons and dark protons the scientists propose might also have antimatter counterparts — dark anti-electrons and dark anti-protons. When these particles collide with their counterparts, they would release gamma rays, the most energetic form of light, which telescopes should be able to spot. Furthermore, dark atoms might also have formed clouds of dark plasma, ripples in which might have influenced the formation of the early universe and thus have visible effects on large-scale cosmic structures that exist nowadays.