Dark Matter - An Identity Crisis?

by Nikolas Iwanus

We don’t know what the universe is made of...

The atoms and molecules that make up the visible portions of what we do see can only be, at best, a fraction of what is actually out there according to multiple independent lines of evidence. There’s no way about it, either gravity doesn’t work like we think it does on the length scales of galaxies, or we’re simply under-counting the mass, that is, the light-to-mass ratio is less than one and there is a lot of Dark Matter out there.

Complicating the matter is we now know that the missing Dark Matter can’t just be ordinary matter that isn’t glowing bright enough to see, like brown dwarfs or neutron stars. If Dark Matter were just these ‘Massive Astrophysical Compact Halo Objects’ or MACHOs, than we should still be able to see their gravitational effects; like light rays from visible galaxies being bent by the MACHOs, weakly distorting their shapes. The effect is small on individual galaxies but by looking at many objects for a cumulative effect of these distortions we can comfortably rule out MACHOs making up anything more than a minority of the missing mass.

A popular class of Dark Matter models that have seen the most study are Weakly Interacting Massive Particles or WIMPs for short. WIMPs seem to fit the bill for Dark Matter perfectly. They’re weakly interacting, compared to other forces like electromagnetism, so the main effect we see is their gravitational effects without seeing them with light today. However in the high energy and density state of the early Universe, WIMPs were able to readily interact with the photons, protons, electrons etc.

From these early Universe interactions we can calculate the abundance of these Dark Matter particles as they stopped interacting, freezing out as stable particles. Miraculously for WIMPs we get the correct measured mass abundance we see today from data collected by the Planck satellite, summarised by the mass parameter: Ω_M = 0.3089±0.0062.

Although they are weakly interacting today they are not non-interacting. Astrophysicists expect to see signals from places were Dark Matter densities are high, like the Galactic Centre or Dwarf Galaxies, where two Dark Matter particles ‘annihilate’ each other and release energy in the form radiation like gamma rays.

In my research I've been writing cosmological simulation codes that take into account the effect of these weak interactions as the galaxies grow from the big-bang until today and study how these models would affect observations.

Density images of the Dark Matter (Red) and Gas (Blue) of a simulated universe with these interactions turned off (Left) and on (Right) where the Dark Matter Annihilation produces heating radiation and stops gas from accumulating in small Dark Matter Halos. Image - Nik Iwanus

Other examples of Dark Matter candidates include Axions, Sterile Neutrinos, Fuzzy Dark matter.