Dark Matter Domination

Dark matter is everywhere.  There is way more of it in the universe than the matter we are made of and interact with.  Yet for the sheer amount of it, we have no way of determining what exactly it is.  It’s as if we didn’t know what air was, and even though we could see it and breathe it, we couldn’t measure it.  The most tantalizing part about dark matter is that we can see the gravitational effect it has, and so we can determine how much of it there has to be.  Some places in the universe have more dark matter than others.  The largest reservoirs exist in the vast clusters of galaxies, which have at least six times as much dark matter as light matter.  But on the slightly smaller scale, dwarf galaxies are particularly interesting.  They have a large dark matter ratio, considering their small size compared to normal galaxies like the Milky Way.

Dwarf galaxies have few stars but lots of dark matter. This Caltech FIRE (Feedback in Realistic Environments) simulation from shows the predicted distribution of stars (left) and dark matter (right) around a galaxy like the Milky Way. The red circle shows a dwarf galaxy like Triangulum II. Although it has a lot of dark matter, it has very few stars. Credit: A. Wetzel and P. Hopkins, Caltech

Recently, the highest dark matter to light matter ratio ever measured was observed in the galaxy Triangulum II, a tiny dwarf galaxy just beyond the outer edge of the Milky Way.   Triangulum II has only 1000 stars, but when Caltech astronomer Evan Kirby measured the velocities of stars orbiting the center of the galaxy, he found that the total mass was far greater than expected.

“The total mass I measured was much, much greater than the mass of the total number of stars–implying that there’s a ton of densely packed dark matter contributing to the total mass,” Kirby says. “The ratio of dark matter to luminous matter is the highest of any galaxy we know. After I had made my measurements, I was just thinking–wow.”

Astronomers discovered dark matter by looking at the orbital velocity of stars in the Milky Way as a function of radial distance.  The faster the stars are moving at a given radial distance, the more mass the galaxy has within that distance. If we do this for the entire galaxy, we can measure the distribution of the galaxy’s total mass as a function of distance from its center.   We can do this in any galaxy to find it’s total gravitational mass.  If we look at the number of stars in a galaxy and their total mass, called the light mass, it should be the same as the total mass measured gravitationally.  But the problem is that the total gravitational mass ends up being 2-3 times as much as the light mass, meaning there is something incredibly heavy and invisible, and it’s concentrated near the outer regions of the galaxy – Dark Matter.

Are there ways to detect dark matter? Maybe.  A leading candidate for what dark matter is made of is what we call Supersymmetric Weakly Interacting Massive Particles (WIMPs).  These are particles that don’t interact with anything, but are more massive than most of the particles in nature while maintaining their stability.  They are predicted to collide and annihilate every so often, producing high energy gamma rays.  If we can find high concentrations of dark matter, like we are seeing in Triangulum II, we can look for these interactions and potentially confirm what it is.

If we can find out the answer to the dark matter question, we will open a doorway to understanding the make-up of a huge chunk of our cosmos.


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