One of the great challenges in cosmology is understanding the nature of the universe’s so-called missing mass.

Astronomers have long known that galaxies are held together by gravity, a force that depends on the amount of mass a galaxy contains. Galaxies also spin, generating a force that tends to cause this mass to fly apart. 

The galaxies astronomers can see are not being torn apart as they rotate, presumably because they are generating enough gravity to prevent this.

But that raises a conundrum. Astronomers can see how much visible mass there is in a galaxy and when they add it all up, there isn’t anywhere enough for the required amount of gravity.  So something else must be generating this force. 

One idea is that gravity is stronger on the galactic scale and so naturally provides the extra force to glue galaxies together.

Another is that the galaxies must be filled with matter that astronomers can’t see, the so-called dark matter. To make the numbers work, this stuff needs to account for some 80 per cent of the mass of galaxies so there ought to be a lot of it around. So where is it? 

Physicists have been racing to find out with detectors of various kinds and more than one group says it has found evidence that dark matter fills our solar system in quantities even more vast than many theorists expect. If they’re right, the Earth and everything on it is ploughing its way through a dense sea of dark matter at this very instant. 

Today, Katherine Freese at the University of Michigan in Ann Arbor, and Christopher Savage at Stockholm University in Sweden outline what this means for us humans, since we must also be pushing our way through this dense fog of dark stuff.

We know that whatever dark matter is, it doesn’t interact very strongly with ordinary matter, because otherwise we would have spotted its effects already.

So although billions of dark matter particles must pass through us each second, most pass unhindered. Every now and again, however, one will collide with a nucleus in our body. But how often?

Freese and Savage calculate how many times nucleii in the average-sized lump of flesh ought to collide with particles of dark matter. By average-sized, they mean a 70 kg  lump of meat made largely of oxygen, hydrogen carbon and nitrogen. 

They say that dark matter is most likely to collide with oxygen and hydrogen nuclei in the body. And given the most common assumptions about dark matter,  this is likely to happen about 30 times a year. 

But if the latest experimental results  are correct and dark matter interactions are more common than expected, the number of human-dark matter collisions will be much higher. Freese and Savage calculate that there must be some 100,000 collisions per year for each human on the planet. 

That means you’ve probably been hit a handful of times while reading this post.  

Freese and Savage make no estimate of the potential impact on health this background rate of collisions might have. That would depend on the energy and motion of a nucleus after it had been hit and what kind of damage it might wreak on nearby tissue. 

It must surely represent a tiny risk per human but what are the implications for the population as a whole?  That would be an interesting next step for a biological physicist with a little spare calculating time. 

Ref: arxiv.org/abs/1204.1339: Dark Matter Collisions With The Human Body