What is Dark Matter?

As mentioned in my previous blog post, visible matter like galaxies, stars, and everything that we see on Earth is only 15% of the matter budget of the universe. Dark Matter makes for the rest of 85% of all matter. So what is Dark Matter? If it’s dark and undetectable, how do we even know it’s there in the first place?! How are scientists devising ways to understand Dark Matter?

How do we know Dark Matter exists?

There are several independent observations that lead to the conclusion that there is some invisible matter in the universe that has mass, and hence exerts gravity, but doesn’t emit light or interact in any way with visible matter. 

So what are these observations? 

1. Speed of stars in spiral galaxies: In our solar system, the majority of the mass is in the center (Sun) and hence planets farther away from the Sun revolve slower than inner planets because there is less gravitational pull on them from the sun, and they would spiral away if they went around faster than they do. Similarly, we expect in spiral galaxies (like our Milky Way) that stars at the edge would go around slower than stars towards the center. However, careful observations show that the stars at the edge of the galaxies are unexpectedly rotating as fast as the inner stars. This shows that these stars towards the edge have gravitational pull from some invisible matter in the galaxy which prevents them from spiraling away. (When we look up in the sky and see stars, how do we know which stars are nearby, which ones are farther, how fast are they travelling, etc? This in itself is a fascinating topic that I hope to cover in a future blog post).
2. Gravitational lensing: Scientists can estimate the mass of a distant galaxy in two different ways: One is by observing all the stars in that galaxy and summing up the estimated mass of various types of stars. They can also estimate the mass of a galaxy by observing the light from galaxies behind this galaxy. Light from galaxies behind this galaxy would have otherwise travelled in a straight line away from us but the galaxy which is in front, bends that light and curves it back towards us. This is called gravitational lensing. The amount of bending depends on the mass of the galaxy. These two estimates of the mass are very different, with the gravitational lensing estimating the mass to be almost 5 times more than what we visibly observe. 
3. Uniform distribution: Scientists have also not observed any unexplained lumps of darkness within galaxies. This indicates that whatever this dark matter is, is distributed pretty uniformly in a galaxy. There might be more in the center of the galaxy but even there it’s not as if large objects are dark and appear like holes within star clusters. We see visible stars spread uniformly and thus this indicates that dark matter is also uniformly spread out amongst it.

How are scientists devising ways to detect Dark Matter?

If you think about it, our instruments such as telescopes detect visible matter by either capturing the photons emitted by that matter (e.g. a distant star) or photons reflected by that matter (e.g. planets reflecting the star’s light) or by interaction with particles in the standard model (e.g. electromagnetic interactions with charged particles).

So now the problem is: What can be something that doesn’t emit light or interact with the standard model particles and is present in massive quantities in our galaxy, and is distributed uniformly? How do we devise an instrument to detect it? Given the various observations listed above, the size range of potential candidates for dark matter can be as large as ‘small’ black holes or as small as subatomic particles. And everything in between! 

Studies of larger objects such as black holes have so far indicated that there aren’t enough of them to explain all the dark matter. They might be responsible for a smaller percentage (~5-10%) of the dark matter budget. This leaves us with a range in the atomic and/or subatomic range of particles, i.e. between 10^-6 eV to 10^11 eV (and ranges above and below that also can’t be ruled out yet!). Note that eV is electron Volt and represents kinetic energy. If you are wondering how mass is being represented as energy, then it’s basically due to E = mc^2. 1 proton is approximately 1 billion eV (or 10^9 eV). An electron is approximately 10^6 eV. Neutrinos are approximately 1 eV and smaller particles called Axions proposed by scientists are 10^-6 eV. So between axions and a heavy element with 100s of protons (e.g. lead atom) we can form the range 10^-6 to 10^11 eV. Also note that we know the total dark matter mass so we can also estimate how many particles of each size can be expected. If the dark matter particles are in the smaller end of the range, then there will be more of them, and hence more ubiquitous also!

Detecting dark particles in the range of 10^6 eV to 10^11 eV (i.e. between an electron and a heavy atom)

Large particle colliders, such as the Large Hadron Collider in Geneva Switzerland, are being used to see if we can detect unexplained energy or momentum during various particle collisions. These observations will indicate the presence of dark particles. 

Another device used to detect dark particles is to create arrays of a heavy atom, such as Xenon, and carefully monitor if anything knocks off the electrons in that atom. Since there are a lot of cosmic rays that continuously knock off electrons in atoms, these Xenon arrays have to be buried deep underground to shield them from other stuff. Remember that dark matter particles just pierce through everything: earth, soil, us, metal etc. because they don’t interact with our protons, neutrons, electrons etc. So they should reach these Xenon arrays buried underground, and we hope they knock off some electrons that we can measure. Nothing so far…

Detecting dark particles in the range of 1 eV to 100 eV

The core reasoning is as follows: Gamma rays are in the range of 1 eV to 100 eV. If we can find all the sources of gamma rays in a particular part of the universe, and if those don’t add up to the gamma rays that we observe, then that unexplained source of gamma rays can be dark matter. Ongoing observations from the Fermi gamma ray telescope are doing this. A large busy galaxy like the Milky Way has lots of sources for gamma rays and so it’s slightly easier to observe other smaller satellite galaxies to see if there are unexplained sources of gamma rays. If we do find this, then at least we know that the dark matter particles are in a smaller eV range and future instruments can be devised to measure dark particles in those ranges. 

Detecting dark particles in the range of smaller than 1 eV

I haven’t found much research in this area. My understanding is that scientists want to rule out the other larger ranges above before figuring out ways to build instruments to detect such small energies. We do know that the Neutrinos, which are in the range of 1 eV, are not dark matter because they travel too fast and don’t stay clustered in a galaxy to explain the gravitational effects of dark matter. So it’s either other kinds of neutrinos or axions.

So, all in all, scientists are figuring out all these possibilities and this remains an open problem in physics. I am personally very excited about this. Explaining the mysteries of dark matter over the next 10, 20, or 30… years will be one of the most exciting events in all of physics. 

My personal fantasy (i.e. a hypothesis without any scientific basis) is that the dark matter is just like visible matter particles (i.e. the Standard model) except that the dark electron, the dark proton, the dark neutron etc. are about 5 times heavier (or they are the same size as corresponding visible particles and that there are 5 times as many of them). And that they do combine with each other to form dark atoms and dark molecules much like visible atoms and molecules. And that there are dark stars, dark planets, and intelligent beings made up of dark cells. Weirdly, in some sense, this can also explain the Fermi paradox (‘Where is everyone?’). Intelligent civilizations, which came millions of years before us, exist all over the universe and we are just not able to ‘see’ their dark artifacts such as spaceships and other megastructures…

Cheers,

Abhi Khune

Abhiram Ganesh Khune