What Dark Matter Reveals About the Structure of the Universe

by JOEL PRIMACK

• • ABOUT 2 YEARS AGO

Dark matter is the vast majority of the mass of the entire universe.  It’s the mass that holds all galaxies together, and in fact, led to the formation of galaxies.  And it also holds clusters together and it made the most important contribution to the organization of the structure of the universe.  

We already know that the dark matter is cold.  I invented this terminology back in 1983, calling the dark matter hot, warm or cold depending on how rapidly it’s moving in the early stages of the Big Bang.  Hot if it’s moving at nearly the speed of light, cold if it’s moving so slowly that its motion hardly matters, and warm is an intermediate case. 

We know that the dark matter has to be pretty cold, but it could be a little bit warm.  And that would make a great difference to what we call small scale structure, the amount of satellite galaxies and things like that.  We don’t yet know the real nature of the dark matter beyond that it’s pretty cold.  

Being pretty cold is enough to allow us to predict in great detail the large scale structure of the universe, the organization of the galaxies and to some extent the satellites of the galaxies.  But the small scale structure of the universe really depends in more detail of the nature of the dark matter.  Also, the dark matter can possibly interact with itself and annihilate and two dark matter particles come together and then make a lot of other stuff.  And this could have played an extremely important role in the early universe, and it could still be producing effects that are sensitive detectors in space and on the ground can find experimentally.  

We haven’t yet seen clear evidence for any of these things, although there are a number of experiments that are reporting tentative detections.  So, it feels very much like we’re on the verge of major breakthroughs in trying to understand the nature of the dark matter.  If we finally do figure out the nature of the dark matter, we will then have a single unified picture of the origin and evolution of the entire universe.  One that scientists all over the world have contributed to and that can become the basis for a shared origin story that could possibly solidify the bonds of humankind.  We’ve never had a single picture, thoroughly supported by scientific evidence, and we’re coming close to it now.  

So I think we scientists are feeling very hopeful that we’re about to cross this threshold and have a complete understanding of the origin and the evolution of the universe.  And of course, we’re also coming to a much better understanding of the evolution of life.  So these last decades of the 20^th century and the first decades of the 21^st century are a real turning point, I think, in our understanding of how we got here. 

In Their Own Words is recorded in Big Think's studio.

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