by Alexander Bastidas Fry
The nearly invisible components of nature such as cells or atoms can only be seen with the aid of tools. If you see a cell with a microscope there exists a physical and philosophical stratification between your perception, your eye, the optics of the microscope, and the observed cell. If you see an atom on a computer monitor rendered from data from an atomic microscope then the layers of complex stratification between you and the atom are monumental. What can we truly know about the nature of things which can only be observed through tools? I would argue quite a lot. Dark matter will always remain isolated from basic human perception, but we can know it through tools or imagination.
Imagine a sea of particles gliding through you unnoticed; this is dark matter. Imagine anything, and dark matter doesn't stop for it. Dark matter doesn't interact strongly with earth, fire, wind or water. There are many particles that have elusive existences similar to dark matter like photons or neutrinos. Unfamiliarity with these known particles doesn't hinder your ability to imagine dark matter: even these particles were not discovered without stratification between human perception and the thing itself. Imagine bits of dark matter passing through you brain at this moment, every moment, because it probably is. And if it is, but it never interacts with you in any way, does it matter?
The truth is that we don't know exactly what dark matter is. It may not be a particle at all. It could have other curious kinds of existence, but those ideas are disfavored. We know a few basic things about dark matter. It interacts very weakly with matter, if at all, so it is not very luminous. It is stable over the current age of the universe. It has a speed that is much smaller than the speed of light. And finally, we know it is not made of the regular matter that makes up galaxies, planets, stars, or you.
Dark matter outweighs regular matter by a factor of five in our universe. Dark matter's presence in our universe is inferred through its observed gravitational effects. It forms a dark cosmic scaffolding that hosts the galaxies we do see—the conspicuous galaxies we can observe are always nestled in the heart of much more massive diffuse dark matter clouds or halos. Dark matter controls the architecture of the universe at the largest scales because it dominates the gravity of the universe and gravity is is the only significant long-range force. A myriad of robust observations lead us to the conclusion that dark matter is a necessary component of the universe. Dark matter is the simplest solution to the complex observations. And yet Dark matter is one of the great unsolved problems in physics. Theories for detection of dark matter are abundant. Do we even have a chance to ever catch dark matter?
We can know dark matter with tools. The current generation of dark matter experiments has already placed strong constraints on what dark matter is not. There is a lot of exploration to be done. The hunt for dark matter can be broken into three categories: creation, direct detection, and indirect detection.
The creation of dark matter would be ideal, the way world peace would be ideal. A particle accelerator, like the Large Hadron Collider for example, could hypothetically create dark matter particles. The new particle would right zip out of the machine ignoring the detectors, the magnetic fields, and the zoo of other particles present. So, the signature of dark matter would be missing energy and momentum inside the detectors. Nothing would be something, but so far nothing—nothing has been seen. Particle physicists have hypothesized a dark matter candidate particle known as the Weakly Interacting Massive Particle or WIMP. The WIMP is an appealing candidate for dark matter because it would naturally satisfy all of the current constraints and it is motivated from fundamental theoretical considerations. A WIMP could come in many varieties. A particularly strong candidate is the neutralino which is the supersymmetry partner of the neutrino. Supersymmetry is a theoretical speculation that predicts every particle in the Universe would have at least one super partner (for example there could be five total Higg's Bosons). The super symmetric counterparts of most particles would be unstable, but the very lightest weight super partner would be stable for the age of the universe and it could be the dark matter. The best shot at discovering supersymmetry right now is the Large Hadron Collider which is being restarted this year at the highest energies ever.
Direct detection of dark matter involves spotting the interaction of dark matter with regular matter in the laboratory. Dark matter's feeble interaction with regular matter, if it occurs at all, would be rare. For some theories there would be an occasional collision of a dark matter particle, perhaps a WIMP, with the nucleus of an atom inside detectors. We are just beginning to build experiments with the necessary sensitivity to find some kinds of dark matter in deep underground labs around the world. Direct detection experiments must be shielded from stray particles that come from distant astrophysical objects and from the radioactive decay of atoms on Earth. Directly detecting dark matter is a waiting game. Only non-detections have been made so far. Direct detection experiments must make assumptions about what kind of thing dark matter is, so theory dictates what kind of experiment and what conclusions can be drawn.
Indirect detection of dark matter relies upon the possible self-annihilation of dark matter in outer space. The idea is that when two dark matter particles meet they may annihilate and ultimately covert their mass (via Einstein's equation E=mc2) into photons. These energetic photons could be observed by telescopes such as the Fermi Gamma Ray Telescope. So far no convincing signal of excess photons has be seen in the sky, but many mysterious signals have been seen. Skeptical reasoning would argue that every signal seen from space so far is consistent with statistical noise or standard astrophysical sources: space is filled with massive collapsing stars, pulsars, black holes, and exotic objects that emit high energy photons that could mimic the effect of annihilating dark matter. Either dark matter isn't annihilating or our telescopes are right on the edge of detecting it.
The possibility remains that dark matter really doesn't have any interaction with regular matter: particle physicists will never create, observe, or detect any sign of it. This would leave astronomical observations of its gravitational signatures as the only way forward. It would leave the door open to the so called 'dark sector' where some traditional rules of physics could be largely thrown out. In my research I have explored one variety of strange dark matter: the possibility of self-interacting dark matter. This particular form of dark matter would scatter off of itself in dense galaxy cores, but would be a largely unnoticed phenomena in other realms of astrophysics. Naturally it would fit all cosmological theories at large scales. Through supercomputer simulations of galaxies we have noticed that self-interacting dark matter could even ameliorate some issues we have had in trying to match simulated galaxies to real galaxies.
Illuminating the problem of dark matter is proving as subtle as its existence. The current direct detection experiments have placed upper bounds on the interaction rate of dark matter with regular matter and indirect detection observations have place upper bounds on the self-annihilation rate of dark matter. Science is more in the business of saying what is not rather than what is. We will continue to place stricter constraints on the properties of dark matter, but if nature is unkind we will never break through to actual detection. We will never grasp dark matter in our hands no matter how long we wait.