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An important of the cosmic puzzle yet I can't help but fear it is an indication of our ignorance, less dubious than it's sibling Dark Energy but it isn't well understood and often poorly explained to the lay person. The existence of dark matter relives upon our models being correct and then the inferences that we draw are also correct. Observations in space will always yield and indirect proof of its existence while a laboratory test is ultimately what we need. The evidence from space is strongly in favour of it, but that does rely upon our models being correct. Using an Occam's razor type argument we can deduce that the inclusion of dark matter is the simplest explanation for the data.

 

In this article I will cover a couple of key points from history and spend the rest talking about what we think it is and how we think it behaves.

 

 

Rotation Curves

The idea of Dark Matter was postulated as a method which accounts the missing matter in galaxy rotation curves. Fritz Zwicky (in 1933) is credited with being the first to notice that the edges of galaxies appeared to be rotating faster than he expected from using simple mass-to-light ratios with the Virial theorem. The total amount of light from the edges of galaxies suggested that the total mass of the galaxy should be lower than the mass inferred from the speed of rotation. This implies that some mass was missing, on top of that it must be dark in the sense that it emits no light. This phenomenon went uncorroborated for 40 years until the work of Ruben and Ford which came out in agreeance for galaxy rotation curves showing a deficiency of mass.

 

There are essentially two possibilities here: (1) that our understanding of gravity / our model of galaxies is incorrect and hence there is no missing matter as we have our equations wrong, or (2) that our equations are pretty much correct (certainly, they can always be improved) and there is some form of ''matter'' that we can't see. For the remainder of this article I'm going to work on the basis that option 2 is correct.

 

The rotation curves suggest there is some missing matter and the first obvious thing to suggest is that there should be a lot of dust, dead stars, or other blobs of ordinary (very scientific, yes) that we can't detect so easily. Ordinary matter is something that we call Baryonic, the precise details are slightly far from where I'm hoping to go in this article. We are familiar with matter, such as air or water, having temperature or pressure; the molecules in water can bounce off each other and they can even absorb and re-emit light. That is to say that we expect some sort of light signature (not necessarily optical light) from the matter. However, our observations don't see enough of a signature from space to indicate that there is enough ordinary matter to explain the rotation curves.

 

 

Galaxy clusters

 

Further studies from the movement of galaxies also suggests an abundance of dark matter between galaxies which is causing them to move to gravitate towards each other and speeds which are faster than expected. It is possible to infer the amount of matter in deep using a technique known as weak lensing. This method looks at the distortion of light in an observation and determines how gravity and bent the ''flight path'' that light has taken from its source to our telescopes here on Earth. The greater the bends in the path then there more mass there is: bigger masses have a stronger gravitational force, and using modern theories of gravity we say that larger masses cause a greater bend in the ''fabric'' of space (or, more accurately, space-time).

 

 

Cosmic Microwave Background/ Baryon Acoustic Oscillations

 

The greatest source of cosmological data is probably the Cosmic Microwave Background (CMB). The CMB is a glimpse into the Universe's past, it is the oldest thing we can see and hence light has travelled the furthest to reach us here on Earth (don't worry if that doesn't make complete sense, it isn't wholly necessary for this article). What we think happened was that the matter in the Universe endured a period when light scattered with the electrons in the atoms, the last time that such a period existed for the whole Universe was said to be the time of Recombination (roughly 380,000 years after the Big Bang). The light signature produced from this scattering process is what we call the CMB, and is what we detect here on Earth all these billions of years later. That's the idea of the CMB in a nutshell, a little bit rough but hopefully its enough for the next (unfortunately complicated) part...

 

The light of the CMB can be characterized by particular patterns that are well understood, sometimes physicists talk about it being Gaussian. Basically we are looking for correlations in the data and trying to figure out if these correlations match the models we've created, we can create something called a power spectrum. The important thing here is that it is an expression of the correlations in the data. In particular we want to look at the sky at different angles and to see if the light coming from different parts of the Universe is correlated in a way that we expect. This type of correlation is called an angular power spectrum.

 

Analysis of the CMB's angular power spectrum reveals something that we call Baryon Acoustic Oscillations (BAOs). Concisely: BAOs are a type of diffusive damping that arises from the interaction of photons (light) and baryons (ordinary matter, also plasmas in case you wondered). What is happening is that there are compression waves in the matter at this time (like sound waves) and these waves produce a signature in the data that we can call a correlation. That is to say that the light from this matter that we detect here on Earth all of these billions of years later shows a strong indication of compression waves. The length of the waves gives us an indication of the abundance of dark matter.

 

In order to fully account for what's happening at this time in the Universe we cannot only rely upon ordinary (baryonic) matter, we actually need some form of dark matter. However, dark matter is so strange that it only interacts with normal matter via gravity. The two types of matter attract each other under gravity but otherwise they don't know each other exists. Ultimately we do suspect that dark matter will interact with ordinary matter via some process other than gravity but we haven't been able to detect anything yet. This gives rise to different ''flavours'' of dark matter: these are theorized types of dark matter that interact with ordinary matter with varying degrees of interaction.

 

 

Types of Dark Matter

 

There are three main categories of dark matter, where each category has a different degree of interaction with other types of matter.

 

Cold Dark Matter (CDM) - does not interact with ordinary, or even itself, at all. This type of matter has no pressure or temperature and is also the flavour of dark matter which is favoured by observational data. Currently, CDM is is a key component of the Standard Model of Cosmology (the \Lambda-CDM model) and makes up around 22% of the total energy budget of Universe. Of course, it is still theory and we do actually suspect that whatever dark matter actually is that it will interact with ordinary matter.

 

Hot Dark Matter (HDM) - All have unsurprisingly similar names. This type of dark matter is suggested to have strong interactions with ordinary matter and hence it suggests that dark matter should produce an obvious electromagnetic (light) signature, but this is something we see in the data.

 

Warm Dark Matter (WDM) - Something in between the two, although I'd say it is closer to CDM. This type of dark matter interacts with ordinary matter but it doesn't provide a very strong signature. It is perhaps the most realistic candidate: CDM is a very simplistic type of matter that seems almost unlikely to be true, yet HDM is closer to our everyday understanding of matter yet the data rules it out. WDM which is close to CDM in behaviour is probably correct.

 

(Caveat: there are also exotic types of dark matter that have been suggested but that's for another day)

 

 

Computer Simulations

 

The observations of the CMB light is fed into computer simulations that model how the Universe should evolve. A common technique is to use simulations with theoretical particles in a box and see how they behave under gravity. If gravity is the only force then we expect the computer particles to attract themselves to each other. This is definitely the case and the patterns that emerge are fairly similar to what we see in the real Universe. For further reading on this pattern look up Large Scale Structure and/or the Cosmic Web.

 

The particles in these simulations are essentially have the same characteristics as the CDM flavour of dark matter. That is to say that they only only interact via gravity. They are said to be: collisionless (non-interacting, no scattering), dissipationless (don't cool by photon radiation), and non-relativistic (speed is much less than that of light). As mentioned above, this is a very idealized type of matter that is almost completely 'dead'. It doesn't do much except interact with others via gravity. The reality is probably going to be a bit different, so not quite collisionless and dissipationless; basically, we expect that dark matter will interact with other types of matter in more ways than one.