COSMOS 3D dark matter map
Image: NASA/ESA/Richard Massey (California Institute of Technology)

First identified in the 1930s, “dark matter” remains as elusive today as ever. It’s what keep scientists up at night. What is known is that it affects the movement of galaxies and other celestial bodies. While physicists can measure its effects, what it is made of is still undefined. Not that physicists aren’t trying or they lack ideas, but none seem to completely explain this phenomenon.

Top 9 Explanations

WIMPs

The current front runner for explaining this phenomenon, weakly interacting massive particles (WIMPs) encompass many dark matter particles, several of which are contained in this list. They are thought to interact with one another through the weak force of radioactive decay and only have 1-to-1000 times the mass of a proton. Despite looking for them on both earth and in space, physicists are having a hard time finding clear connections to WIMPs and the effects of dark matter.

SIMPs

Physicists have a decent idea of how much dark matter exists based on galaxies observation, although they have never actually detected it. Observations of inner regions of galaxies do not align with their models. These models often assume that dark matter does not interact with itself. To remedy this, strong interactive massive particles (SIMPs) were introduced; they seem to eliminate the discrepancy of these inner versus outer galaxies. This would also explain the photon signal that comes from galaxy cluster.

Neutralinos

Neutralinos come from the Theory of Supersymmetry. Basically, this theory says that every particle has a “super” partner that fills the holes in the standard model. But it has yet to be observed.

Some of these “super” partners, like those of the Z boson and photon, have characteristics similar to dark matter. Of these, the neutralino is most likely and would solve two major issues in physics:

  1. Tell us the identity of dark matter
  2. Give us proof of supersymmetry

However it also opens up a ton of new questions, essentially telling us there are a lot more particles waiting to be discovered. 

CMS Higgs-event
CMS Higgs-event. Image: Lucas Taylor / CERN

Sterile Neutrinos

Neutrinos are odd enough: They’re almost mass-less, they have the ability to shape-shift, and they can pass through an entire planet without hitting anything. Despite their strangeness, they have an even more strange counterpart, the sterile neutrino.

These sterile neutrinos would be virtually unresponsive to particles around them. So much so that they may never collide in the entire age of the universe. This aversion to interaction would make them almost impossible to detect, but they may decay into photons, a particle we can easily detect. Hopefully, the new ASTRO-H Japanese telescope will help shed some light on the decaying sterile neutrino hypothesis.

Axions

First discovered in the early 1980s, axions are experiencing a surge in popularity, with many believing they could be the answer to dark matter. The University of Washington’s Axion Dark Matter Experiment (ADMX) is searching for these particles by using a strong magnetic field to try and turn them into photons. With a greater ability to detect these particles, scientists are dusting off this theory and searching for new ways to detect them.

Mirror World Dark Matter

The theory basically says that dark matter exists in a separate universe with similar elemental particles to ours; however, they only exert the force of gravity. This works well with what we do know about dark matter (its gravitational pull), but does little to satisfy any why or how questions.

Asymmetric Dark Matter

Asymmetric dark matter is basically just the antimatter-matter scenario (i.e., cancel each other out). In this theory, dark matter collides with anti-dark matter, thus leaving behind what we perceive currently as dark matter.

Extra Dimensional Dark Matter

A slight variation of the Mirror World Theory, extra dimensional dark matter would simply exist in a fourth spacial dimension that we have yet to see. This fourth dimension would be too small for us to see a particle’s movements with it; rather, we would see multiple particles with the same charge but different masses. Proving this theory would support String Theory, which needs extra dimensions to work.

Composite Dark Matter

This theory is simply a combination of the above theories. Since none of them fill the bill completely, it might be possible that bits and pieces from various theories all hold clues to answering the question of what dark matter actually is.