QotW: Is dark matter in lumps or like sand?

The invisible stuff is an essential part of our universe, but does it come in lumps or as grains of sand?
02 November 2021


Colourful image of the Universe



Listener Rob asks, "Is dark matter in large lumps or like grains of sand?"


In the classic novella The Little Prince, we encounter our eponymous hero in the middle of the Sahara desert amongst massive sand dunes. He says “What is essential is invisible to the eye”. Just like in the quote, dark matter is an invisible substance that is essential to the makeup and evolution of our universe. To answer Rob’s question, Iacopo Russo asked Durham University professor Francesca Day, and University of Chicago physicist Jacques Pienaar for help...

Fran - Most physicists think Dark Matter exists as individual particles - so much, much smaller than grains of sand. We don't know the size of these Dark Matter particles, but in most theories they would be over a thousand billion billion times lighter than a grain of sand.

Iacopo - We are all familiar with common or normal matter: it’s the stuff that makes up anything around us, like sand, but also rocks, cars and even our bodies. We can see this stuff directly by shining light on it and seeing how it interacts. In contrast, dark matter is stuff that does not interact with light at all. We know it’s there because it explains so much of the way our universe works, but we can’t observe it directly. So why do physicists think dark matter stays as individual particles? I asked Jacques Pienaar, a physicist from the University of Chicago.

Jacques - What current evidence tells us about dark matter is that it interacts very weakly through the electromagnetic force, which is the force that holds objects together in our everyday life. This is also the force holding together smaller objects in space, such as asteroids. But once we move beyond the scale of our everyday interactions, say to the size of planets or stars, the gravitational force becomes the dominant force holding these objects together. The implications for dark matter are that we would not expect lumps in the scale of asteroids or even grains of sand. Dark matter would exist as a cloud of individual particles, which do not interact strongly enough to form clumps.

Iacopo - So, does this mean you cannot make sand castles out of dark matter? Francesca Day has something to say about this.

Fran - This doesn't mean that Dark Matter particles are spread out evenly throughout the universe. Because they do attract each other via gravity. This means that Dark Matter clumps together into dense regions. Galaxies like our own Milky Way form in these denser regions of Dark Matter.

Iacopo - Wait, so: can dark matter form clumps or not? Jacques Pienaar clarifies.

Jacques - What is possible is that when there are large enough clumps of dark matter, i.e. on stellar scales, that the gravitational force becomes strong enough to form very large dark matter objects analogous in some sense to how regular stars are formed out of clouds of hydrogen gas.

Iacopo - Now wouldn’t The Little Prince be happy to learn that this invisible, dark matter was essential to the formation of the very galaxy where we all live in? Next week we are going to run some juicy numbers to answer this question from listener Jodie:

Jodie - Which has the lowest carbon footprint, a punnet of tomatoes from the supermarket or those grown in a grow bag in my garden?


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