DARK MATTER

 

Here’s the thing about dark matter — it’s really easy to understand. All it responds to is gravity, and we understand gravity extremely well. It is much easier to understand than ordinary matter that can respond to electromagnetic and chemical processes.

So given an initial state at early times as seen in the Cosmic Microwave Background, it is pretty straightforward to predict the evolution of matter under the assumption that it only responds to gravity. The behavior is quite nonlinear so it requires numerical simulations, but these are routine nowadays.

This means that it’s very easy to predict where the dark matter is in the Universe, if it is mass that only responds to gravity (on large scales) as all evidence suggests. Since dark matter makes up more than 80% of the cosmic mass, these predictions are surprisingly accurate.

This shows a simulation of the evolution of dark matter from about 1.5 billion years after the Big Bang (z=4) to today (z=0):

Dark matter collects into larger and denser structures as time evolves, because gravity attracts. The pattern of filaments, sheets, and nodes is known as the Cosmic Web. Given a particular cosmological model specifying how much dark matter and dark energy there is, the Cosmic Web can be predicted very accurately.

While we can’t see dark matter itself, we can see its effects on all kinds of scales, in all kinds of circumstances, for all kinds of objects. So we know that the above model for where dark matter is, is pretty close to correct.

The only thing we are not sure about right now is what the dark matter particle actually is. But this is almost a triviality at this point — there is such an incredible preponderance of evidence constraining its properties, it is essentially impossible to develop an alternative model that can so simply and elegantly explain the vast amount of varied observations as straightforwardly as dark matter.