One of the most amazing scientific findings of the last few decades is that we don’t know what makes up most of the universe. The matter that we are made of, and that everything we see is made of, only constitutes a small part of all there is. We know from our observations that there must exist additional matter that does not emit light, and that has been called “dark matter.” Exactly what constitutes this dark matter is not yet known. It is widely believed to be some type of particle, one that interacts very weakly and that in particular doesn’t interact with photons, the carriers of light.

Even though we cannot see it, dark matter makes itself noticeable through its gravitational pull, which can be inferred through a variety of methods, for example gravitational lensing and the motion of visible mass (stars) in the gravitational field of the dark matter. Galactic rotation curves were the first evidence that there must be something more pulling on stars that go around the center of galaxies than these stars themselves. The Milky Way is today believed to be embedded in an approximately spherical dark matter halo.

Our Milky Way, like most galaxies, is surrounded by an approximately spherical halo of dark matter. Our sun moves through this halo.

Our solar system goes around the center of the Milky Way in an orbit that takes some 220 million years to complete. We therefore move into the dark matter which ought to move around the galaxy in a non-rotating fashion, and hence dark matter particles pass (rapidly) through the Earth all the time. The probability that a dark matter particle interacts with the matter that we are made of is small, but every once in a while it ought to happen.

There are now many experiments under way looking for these rare interactions between dark and visible matter. The experiments consist of large tanks that contain certain atoms with desirable properties. Different experiments use different atoms, but they all search for the tiny recoil that an atomic nucleus would get from a dark matter particle. The detectors then either measure the heat created by this interaction, or the light that some atoms emit following the interaction, called “scintillation.”

Photomultipliers in the dark matter detector LUX. Credits: C.H. Faham.

The dark matter particles also move through your body and sometimes they interact with one of your atomic nuclei. The typical energy deposited by a dark matter particle is of the order keV. This is very small for nuclear energies, and thus the nuclei themselves are not affected, they just wiggle a bit. But it is an energy much larger than the energy necessary to break molecular bonds, which is typically about a factor 1000 smaller. Dark matter particles thus can damage the bond structure of molecules inside your body. A few percent of the atomic nuclei in your body is DNA, so just by chance sometimes dark matter will damage your DNA. Most of the time this happens the damage will be repaired or the cell will die. But sometimes it won’t; sometimes, the damaged DNA can live on and reproduce.

So, yes, dark matter can give you cancer. The question is though, how likely is this to happen? Not very likely it turns out. The dark matter detectors presently operating contain something between 50 and 250 kg of mass, so for a theoretical physicist that’s the average spherical cow. (Or even a smaller-than-average cow.) These detectors have an expected number of events between a handful and some thousand per year. The number of expected detections is smaller than the number of actual interactions due to detector inefficiencies, but from this you can already estimate that your body will probably interact with dark matter something between ten times and a few thousand times per year.

You can find more accurate numbers in an interesting paper by Katherine Freese and Christopher Savage. Their calculation leads to about the same result as our sloppy estimate: we interact with dark matter in a frequency between once every few days to a few times a day. In the paper you find more details, for example that the interaction is most likely to occur with oxygen nuclei. The exact number of interactions of course depends on the type of dark matter particle that our Universe contains (which is still unknown), so there is significant uncertainty in the estimate.

When it comes to the impact on your health, nobody really knows how to estimate the probability of getting cancer from damage to certain molecular bonds, but one can compare the damage due to dark matter with that due to cosmic radiation, mostly muons. Not only do muons deposit more energy into your body, they are also more numerous. Freese and Savage estimate that your lifetime exposure to dark matter just about equals the potential damage due to cosmic radiation you get in a second! So when it comes to cancer risks, dark matter is pretty much the last thing to worry about. Not that there was much you could do about it anyway.

That dark matter can break apart DNA may even be a good thing.

One of the dark matter detectors, by the name of DAMA, has been measuring a signal for many years now, and this signal changes periodically over the year. One would expect such an annual modulation from a true dark matter signal due to the motion of the Sun through the dark matter halo. Unfortunately, the properties of dark matter particles that would produce the right signal to explain the DAMA result have been ruled out already by other experiments. The signal that DAMA is getting is very well confirmed — they are clearly seeing something. But because of the conflict with other experiments many researchers think DAMA sees something else than dark matter. What it is that DAMA’s seeing, though, they don’t know.

One way to resolve this confusing situation would be to measure the direction from which the particles come. That is because we do not know of any other signal that would come in like the headwind of dark matter particles from our motion around the center of the galaxy. However, the presently existing detectors are not good for this because they were not designed to measure the direction of recoil and are not very sensitive to the exact location of the interaction either.

But there is neat way to track the direction of incoming dark matter particles which was proposed by Drukier et al. in a 2012 paper. Their idea is to use the breaking of straight (not curled up), closely spaced, DNA strands to reconstruct the recoil of an atom hit by dark matter.

Their experiment works as follows: wait until a dark matter particle interacts with a thin layer of gold, and kicks out one of the atoms. The gold atom carries on much of the momentum of the dark matter particle, so it will continue into about the same direction. Below the gold layer there are the DNA strands hanging, and whenever the gold atom hits one, the DNA is likely to break. From the dark matter particle, the gold atoms get about enough energy to break a few hundred of the strands.

Sketch of the proposed experiment. An incoming dark matter particle kicks out a gold atom which goes on to break several strands of DNA.

Now if one knows the sequence of the DNA strands being used, they constitute basically a coordinate system. What one has to do then is sweep up the broken off DNA ends, find out where they were broken, and reconstruct the path of the gold atom, thereby revealing the direction from which the dark matter particle came.

It didn’t really become clear to me why one should use DNA in particular. It seems to me any long enough polymer with a known and not too repetitive sequence could be used. The main benefit of DNA I think is that it is a very well-known and much studied molecule and there exists many tools to design, replicate, and measure DNA strands. Additionally, sequence reconstruction techniques (like overlaps) have been highly developed, and the technology to re-sequence it quickly and easily is well in hand.

I like this proposed experiment not only because it could resolve the dark matter mystery but also because it so nicely connects physics with biochemistry and molecular biology. The secret to unlocking the mystery of dark matter, experimentally, might be written in the code of our very existence!