Question: what is dark matter and how does it occur?

Claire Lee answered on 14 Jun 2013:

This is a really great question! The answer is – nobody really knows! (and it’s one of the top “to be answered” questions in physics today).

But to get a bit of an idea about what dark matter is, let’s start with what it does:

* It does not interact through the electromagnetic force (ie it does not have electric or magnetic charge)
* It does not interact through the strong or weak nuclear forces (these are the forces that hold protons and neutrons together inside an atom (strong), and are responsible for radioactive decay (weak) )
* The only way it interacts with normal matter (like everything in the universe we consider “normal”) is through gravity
* It clumps together (because of gravity)
* It is responsible for the overall, large-scale structure of the universe

Without knowing what it actually is, we can’t really say too much about how it occurs, but we do know that a lot of it was created at the beginning of the universe, along with the rest of matter. Now the cool part is that the dark matter started clumping together immediately, while the ordinary matter couldn’t start clumping until about 300 thousand years later. So the dark matter formed these clumps, or structures, and later the normal matter started collecting inside these clumps of dark matter, which is where the galaxies ended up forming. So dark matter really is responsible for what the universe looks like!

As for what it is, well we don’t know, but we have a few ideas. The best idea that we have at the moment is a new kind of particle, one that we haven’t discovered yet, but some of our theories predict them. We’re looking for them already at the LHC, we haven’t found anything yet, but that doesn’t mean we won’t in the future.